catalyst-auto: move git update into main script

All of our config files are doing this already, so move it into the
common code.

tools/catalyst-auto-amd64-experimental.conf

@@ -72,9 +72,6 @@ update_symlinks() {
 }
 
 pre_build() {
-	pushd ${REPO_DIR} >/dev/null
-	git pull
-	popd >/dev/null
 	update_symlinks
 }