Fix gitian version string issue by reverting the GIT_DIR backport commit

We partially revert 8dbd2edd98 by removing
all instances of setting `GIT_DIR` in the gitian descriptors. Currently,
gitian builds cause zcash to have version strings which look like
`Zcash Daemon version v5.3.0-35186b009-dirty`, which is inaccurate.

Setting `GIT_DIR` with a current working directory of
`/home/debian/build/zcash/distsrc-x86_64-linux-gnu` makes the git
invocations in `genbuild.sh` see that directory as the top of the
working tree, which causes git to believe that a bunch of tracked files
were deleted (namely, those not included by the `make dist` step).

This causes `genbuild.sh` to believe that it is working with a dirty
tree, and with the help of `src/clientversion.cpp`, zcash gets a version
string that includes a hash ending in `-dirty`.

contrib/gitian-descriptors/gitian-linux-parallel.yml

@@ -97,7 +97,6 @@ script: |
   create_per-host_faketime_wrappers "${REFERENCE_DATETIME}"
 
   # Create the release tarball using (arbitrarily) the first host
-  export GIT_DIR="$PWD/.git"
   ./autogen.sh
   CONFIG_SITE=${BASEPREFIX}/$(echo "${HOSTS}" | awk '{print $1;}')/share/config.site ./configure --prefix=/
   make dist

contrib/gitian-descriptors/gitian-linux.yml

@@ -97,7 +97,6 @@ script: |
   create_per-host_faketime_wrappers "${REFERENCE_DATETIME}"
 
   # Create the release tarball using (arbitrarily) the first host
-  export GIT_DIR="$PWD/.git"
   ./autogen.sh
   CONFIG_SITE=${BASEPREFIX}/$(echo "${HOSTS}" | awk '{print $1;}')/share/config.site ./configure --prefix=/
   make dist

contrib/gitian-descriptors/gitian-osx.yml

@@ -92,7 +92,6 @@ script: |
   create_per-host_faketime_wrappers "${REFERENCE_DATETIME}"
 
   # Create the release tarball using (arbitrarily) the first host
-  export GIT_DIR="$PWD/.git"
   ./autogen.sh
   CONFIG_SITE=${BASEPREFIX}/$(echo "${HOSTS}" | awk '{print $1;}')/share/config.site ./configure --prefix=/
   make dist

contrib/gitian-descriptors/gitian-win.yml

@@ -113,7 +113,6 @@ script: |
   create_per-host_linker_wrapper "${REFERENCE_DATETIME}"
 
   # Create the release tarball using (arbitrarily) the first host
-  export GIT_DIR="$PWD/.git"
   ./autogen.sh
   CONFIG_SITE=${BASEPREFIX}/$(echo "${HOSTS}" | awk '{print $1;}')/share/config.site ./configure --prefix=/
   make dist