Remove variables exported by Soong

LLVM_RELEASE_VERSION is unused from envsetup.sh and during product
config loading.  LLVM_PREBUILTS_VERSION is still used in envsetup.sh, so
needs to stay.

Delay reading core/clang/config.mk until after Soong's variables are
read so that we don't need a copy of LLVM_PREBUILTS_BASE.

Test: Compare build-aosp_flounder.ninja
Change-Id: I85b5836a2230838c8f3766fb19c6108fc5c9ca50