Revert "ANDROID: Kbuild, LLVMLinux: allow overriding clang target triple"

This reverts commit 4c451dba.

AOSP's distribution of GNU binutils always had a curious target triple
prefix on the binaries.  Now that GNU binutils is deprecated for Android
Common Kernels, we can now remove this out of tree workaround. Now
building Android kernels with LLVM matches upstream (see
Documentation/kbuild/llvm.rst).

Bug: 118439987
Bug: 120440614
Bug: 141693040
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: Iecaa3264a440f795f2f3a44bdf74fe28ad4ed1cc