[llvm] Revert "[APFloat] Add exp functions for single and double using exp/expf implementations from LLVM libc." (PR #197440)

[via llvm-commits]

lntue wrote:

@joker-eph gcc-9 bots are green now, and I'm working on fixing the gcc-7 bot.  Can you hold this for a bit?  Thanks,

https://github.com/llvm/llvm-project/pull/197440