Revert builtins fp16 support: tests do not pass on Mac

Revert "[compiler-rt] [builtins] Support conversion between fp16 and fp128" & dependency

Revert "[compiler-rt] [builtins] Use _Float16 on extendhfsf2, truncdfhf2 __truncsfhf2 if available"

This reverts commit 7a948298.

This reverts commit 1fb91fcf.