ANDROID: remove stale variables from build.config files

LLVM_IAS=1 is implied by LLVM=1 as of LLVM_IAS as of v5.15-rc1 via
commit f12b034a ("scripts/Makefile.clang: default to LLVM_IAS=1").

CROSS_COMPILE is no longer necessary when building with LLVM=1 as of
v5.15-rc1 via:
commit 231ad7f4 ("Makefile: infer --target from ARCH for CC=clang")

There is no need for CROSS_COMPILE_COMPAT as of v5.16-rc1 via
commit 3e6f8d1f ("arm64: vdso32: require CROSS_COMPILE_COMPAT for gcc+bfd")
when LLVM=1 is set. This was backported to v5.15.12.

LINUX_GCC_CROSS_COMPILE_PREBUILTS_BIN was a temporary dependency until
LLVM_IAS=1 was enabled for all architectures. With LLVM_IAS=1 (implied
by LLVM=1), nothing in this directory is used and the variable can be
removed.

It doesn't hurt to respecify any of the above, but they are no longer
necessary.

Bug: 209655537
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I361b05ea9f36da933e6712150650f476b093d0a7