makepkg: remove GITFLAGS support

Supporting git source fragments (branch, commit, tag) is difficult in
conjunction with GITFLAGS usage - particularly with the most common
use cases that reduce the amount of data cloned from the upstream repo.

Leaving GITFLAGS in place an documenting that various git source features
are not supported when GITFLAGS are in used is not an ideal 'solution'.
Instead, remove GITFLAGS support.

Signed-off-by: Allan McRae <allan@archlinux.org>

doc/makepkg.8.asciidoc

@@ -297,9 +297,6 @@ Environment Variables
 **BUILDTOOLVER=**"<version>"::
 	The version of the '$BUILDTOOL' used.
 
-**GITFLAGS**::
-	The options to pass when checking out git sources, replacing the default
-        "--mirror".
 
 Configuration
 -------------

scripts/libmakepkg/source/git.sh.in

@@ -49,7 +49,7 @@ download_git() {
 
 	if [[ ! -d "$dir" ]] || dir_is_empty "$dir" ; then
 		msg2 "$(gettext "Cloning %s %s repo...")" "${repo}" "git"
-		if ! git clone --origin=origin ${GITFLAGS:---mirror} "$url" "$dir"; then
+		if ! git clone --origin=origin ---mirror "$url" "$dir"; then
 			error "$(gettext "Failure while downloading %s %s repo")" "${repo}" "git"
 			plainerr "$(gettext "Aborting...")"
 			exit 1