Revert adding .git-revision unconditionally

This reverts commit 1e534e234b0a92bf06361fa41b7ac8691fdbc769.

We already should have a .git directory if it is managed via Git,
otherwise there is no way to get the Git revision if neither
.git-revision or .git is present.

But having .git-revision _and_ .git present seems very much redundant to
me.

Signed-off-by: aszlig <[e-mail]>
Cc: @bennofs, @Profpatsch
Issue: #17218

nixos/modules/installer/tools/get-git-revision nixos/modules/installer/tools/get-version-suffix

···
 if nixpkgs=$(nix-instantiate --find-file nixpkgs "$@"); then
     getVersion $nixpkgs
     if [ -n "$rev" ]; then
-        echo "$rev"
+        echo ".git.$rev"
     fi
 fi

nixos/modules/installer/tools/nixos-rebuild.sh

···
 # nixos-version shows something useful).
 if [ -n "$canRun" ]; then
     if nixpkgs=$(nix-instantiate --find-file nixpkgs "${extraBuildFlags[@]}"); then
-        revision=$($SHELL $nixpkgs/nixos/modules/installer/tools/get-git-revision "${extraBuildFlags[@]}" || true)
-        if [ -n "$revision" ]; then
-            echo -n ".git.$revision" > "$nixpkgs/.version-suffix" || true
-            echo -n "$revision"  > "$nixpkgs/.git-revision" || true
+        suffix=$($SHELL $nixpkgs/nixos/modules/installer/tools/get-version-suffix "${extraBuildFlags[@]}" || true)
+        if [ -n "$suffix" ]; then
+            echo -n "$suffix" > "$nixpkgs/.version-suffix" || true
         fi
     fi
 fi