remove dead vulnerable code in ext_gd.cpp

remove dead vulnerable code in ext_gd.cpp

Summary:
Based on PHP Fix bug #78256 (heap-buffer-overflow on exif_process_user_comment), which doesn't affect HHVM because this code is dead.

EXIF_USE_MBSTRING is never defined so this is dead code (it doesn't even compile when I tried forcing EXIF_USE_MBSTRING = 1). Given that the dead code contains a potential vulnerability (CVE 2019-11042), it's probably safer to delete it completely and not risk someone enabling it later.

I kept the test case from https://bugs.php.net/bug.php?id=78256 even though no fix was necessary, to prevent potential regressions in the future.

Reviewed By: alexeyt

Differential Revision: D19898317

fbshipit-source-id: 292a74bc818518d18d824fb8c08bbd9eed6b2c0f