[gitian] Default reference_datetime to commit author date

contrib/gitian-descriptors/gitian-linux.yml

@@ -18,7 +18,6 @@ packages:
 - "binutils-gold"
 - "ca-certificates"
 - "python"
-reference_datetime: "2016-01-01 00:00:00"
 remotes:
 - "url": "https://github.com/bitcoin/bitcoin.git"
   "dir": "bitcoin"

contrib/gitian-descriptors/gitian-osx-signer.yml

@@ -6,7 +6,6 @@ architectures:
 - "amd64"
 packages:
 - "faketime"
-reference_datetime: "2016-01-01 00:00:00"
 remotes:
 - "url": "https://github.com/bitcoin/bitcoin-detached-sigs.git"
   "dir": "signature"

contrib/gitian-descriptors/gitian-osx.yml

@@ -27,7 +27,6 @@ packages:
 - "python-dev"
 - "python-setuptools"
 - "fonts-tuffy"
-reference_datetime: "2016-01-01 00:00:00"
 remotes:
 - "url": "https://github.com/bitcoin/bitcoin.git"
   "dir": "bitcoin"

contrib/gitian-descriptors/gitian-win-signer.yml

@@ -7,7 +7,6 @@ architectures:
 packages:
 - "libssl-dev"
 - "autoconf"
-reference_datetime: "2016-01-01 00:00:00"
 remotes:
 - "url": "https://github.com/bitcoin/bitcoin-detached-sigs.git"
   "dir": "signature"

contrib/gitian-descriptors/gitian-win.yml

@@ -21,7 +21,6 @@ packages:
 - "zip"
 - "ca-certificates"
 - "python"
-reference_datetime: "2016-01-01 00:00:00"
 remotes:
 - "url": "https://github.com/bitcoin/bitcoin.git"
   "dir": "bitcoin"