Remove make variables exported from Soong

These have all been strictly checked by Soong, so the values currently
match. Just remove them so that the make variables exported by Soong
will take over.

This also removes some of the GCC reconfiguration, since we haven't
implemented that in Soong. If it becomes useful in the future, we'll
implement it there.

Some things like TARGET_TOOCHAIN_ROOT can nearly be removed, but are
used before the Soong-exported make_vars.mk is loaded. In that case,
added to the clang cflags, so it can be removed once the cflags are
removed.

Others, like LLVM_PREBUILTS_VERSION are loaded even earlier -- in
envsetup.sh before we've even configured Soong. So for now, keep the
Make definition, and continue ensuring it's the same as the Soong
version.

Change-Id: I554b27062ac43610828a8c06d640d919a2dc21ca