ANDROID: remove more stale variables from build.config files

CROSS_COMPILE is no longer necessary when building with LLVM=1 as of
upstream v5.15-rc1's
commit 231ad7f4 ("Makefile: infer --target from ARCH for CC=clang")
which was backported to android13-5.10.

LLVM_IAS=1 is implied by LLVM=1 as of LLVM_IAS as of
upstream v5.15-rc1's
commit f12b034a ("scripts/Makefile.clang: default to LLVM_IAS=1")
which was backported to android13-5.10.

Bug: 209655537
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: Id426f680bba8d478ed300dd0f6551dc419ea345f