Revert "Reland "[compiler-rt][BF16] Provide __truncsfbf2 only when __bf16 is available""

This reverts commit 0729d001.

Buildbot https://lab.llvm.org/buildbot/#/builders/19/builds/11989 failed
with "error: ISO C requires a translation unit to contain at least one declaration [-Werror,-Wempty-translation-unit]"