Don't filter on a non-existent variable.

This used to be used for aarch64
(https://android-review.googlesource.com/#/c/80126/), but has since
been removed and now does nothing. This was then reimplemented more
generally with my_illegal_flags
(https://android-review.googlesource.com/#/c/166405/). Drop
*GLOBAL_UNSUPPORTED_CFLAGS in favor of that.

Change-Id: I3347726dd92f9d3a16783e0d9d3b685a828400b4