[clang] Remove -Wexperimental-float-control.

Summary: Per D62731, the behavior of clang with `-frounding-math` is no worse than when the rounding flag was completely ignored, so remove this unnecessary warning.

Reviewers: mibintc, chandlerc, echristo, rjmccall, kpn, erichkeane, rsmith, andrew.w.kaylor

Reviewed By: mibintc

Subscribers: merge_guards_bot, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71671