Merge #15584: build: disable BIP70 support by default

BIP70 is removed in "merge #17165: Remove BIP70 support  (#4023)".
So, this commit contains only some follow-ups to unify codebase

e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 doc: specify protobuf as optional in build docs (fanquake)
376f4929f8f75011b72b2f9c3164980db482278a build: disable BIP70 support by default (fanquake)

Pull request description:

  Disable BIP70 support in the GUI by default for `0.19.0` (for eventual removal in `0.20.0`?).

  Users who want to compile with BIP70 support enabled can pass `--enable-bip70` to `./configure`.

  I've inverted the current `--disable-bip70` test to instead pass `--enable-bip70`.

  Tested configurations on `macOS` (`protobuf` installed with `brew`).
  Protobuf available and `./configure`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = no
  ```

  Protobuf available and `./configure --enable-bip70`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = yes
  ```

  Protobuf not available (i.e `brew unlink protobuf`) and `./configure`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = no
  ```

  Protobuf not available and `./configure --enable-bip70`:
  ```
  checking whether to build test_bitcoin-qt... yes
  checking whether to build BIP70 support... configure: error: protobuf missing
  ```

  TODO:
  - [x] Remove `protobuf` from other Travis builds
  - [ ] Documentation updates (mention that `protobuf` is now optional)?
  - [ ] Could split release notes into GUI and build

ACKs for top commit:
  laanwj:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0
  elichai:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 Read the autotools changes. awesome that this removes the protobuf requirement.
  practicalswift:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 -- diff looks correct

Tree-SHA512: 7bf87ae8555e24db2da2e89cc4d4e90d09be27499ad386ad65879d05df8f96d9a1384379891ac8963d17728c90e55961560813df97e849e631e2de8c08e210c8

.travis.yml

@@ -232,7 +232,7 @@ after_success:
         FILE_ENV="./ci/test/00_setup_env_win64.sh"
 
     - stage: test
-      name: '32-bit + dash  [GOAL: install] [GUI: no BIP70]'
+      name: '32-bit + dash  [GOAL: install]  [GUI]'
       env: >-
         FILE_ENV="./ci/test/00_setup_env_i686.sh"
 
@@ -276,7 +276,7 @@ after_success:
         FILE_ENV="./ci/test/00_setup_env_mac.sh"
 
     - stage: test
-      name: 'macOS 10.14 native [GOAL: install] [GUI: BIP70 enabled] [no depends]'
+      name: 'macOS 10.14 native [GOAL: install] [GUI] [no depends]'
       os: osx
       # Use the most recent version:
       # Xcode 11, macOS 10.14, JDK 12.0.1

ci/test/00_setup_env_mac_host.sh

@@ -7,7 +7,7 @@
 export LC_ALL=C.UTF-8
 
 export HOST=x86_64-apple-darwin19
-export BREW_PACKAGES="automake berkeley-db4 libtool boost miniupnpc pkg-config protobuf qt qrencode python3 ccache zeromq"
+export BREW_PACKAGES="automake berkeley-db4 libtool boost miniupnpc pkg-config qt qrencode python3 ccache zeromq"
 export PIP_PACKAGES="zmq"
 export RUN_CI_ON_HOST=true
 export RUN_UNIT_TESTS=true

ci/test/00_setup_env_native_cxx20.sh

@@ -6,7 +6,7 @@
 
 export LC_ALL=C.UTF-8
 
-export PACKAGES="python3-zmq qtbase5-dev qttools5-dev-tools protobuf-compiler libdbus-1-dev libharfbuzz-dev libprotobuf-dev"
+export PACKAGES="python3-zmq qtbase5-dev qttools5-dev-tools libdbus-1-dev libharfbuzz-dev"
 export DEP_OPTS="NO_UPNP=1 DEBUG=1"
 export CPPFLAGS="-DDEBUG_LOCKORDER -DENABLE_DASH_DEBUG -DARENA_DEBUG"
 export PYZMQ=true