generate_gni.sh: update_readme() runs git at the end of process, prevent it.

Thanks-to: Georgy Yakovlev <gyakovlev@gentoo.org>

--- a/src/3rdparty/chromium/third_party/libvpx/generate_gni.sh	2021-02-07 12:51:49.438514897 +0100
+++ b/src/3rdparty/chromium/third_party/libvpx/generate_gni.sh	2021-02-21 16:23:38.375724515 +0100
@@ -519,8 +519,3 @@
 
 gn format --in-place $BASE_DIR/BUILD.gn
 gn format --in-place $BASE_DIR/libvpx_srcs.gni
-
-cd $BASE_DIR/$LIBVPX_SRC_DIR
-update_readme
-
-cd $BASE_DIR