Revert adding .git-revision unconditionally

This reverts commit 1e534e234b.

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 <aszlig@redmoonstudios.org>
Cc: @bennofs, @Profpatsch
Issue: #17218

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

@@ -0,0 +1,22 @@
+getVersion() {
+    local dir="$1"
+    rev=
+    if [ -e "$dir/.git" ]; then
+        if [ -z "$(type -P git)" ]; then
+            echo "warning: Git not found; cannot figure out revision of $dir" >&2
+            return
+        fi
+        cd "$dir"
+        rev=$(git rev-parse --short HEAD)
+        if git describe --always --dirty | grep -q dirty; then
+            rev+=M
+        fi
+    fi
+}
+
+if nixpkgs=$(nix-instantiate --find-file nixpkgs "$@"); then
+    getVersion $nixpkgs
+    if [ -n "$rev" ]; then
+        echo ".git.$rev"
+    fi
+fi