remove -ffp-contract

We don't set it in Chrome, so this'll make our local bots a little more
consistent with what we see in Chrome.  Very, very minor stuff.

Bug: skia:9103
Change-Id: Iecedd367780b6a721bc49dfa2d0a7837eb3bd28d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215144
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

BUILD.gn

@@ -279,14 +279,6 @@ opts("hsw") {
   } else {
     cflags = [ "-march=haswell" ]
   }
-
-  # Oddly, clang-cl doesn't recognize this as a valid flag.
-  # If it ever does, it'd nice to move this up with -mavx2 and co.
-  if (is_clang && !is_win) {
-    # This flag lets Clang generate FMAs when it sees a mul-then-add.  It's optional,
-    # but nice to have, generating slightly better code for paths without explicit FMAs.
-    cflags += [ "-ffp-contract=fast" ]
-  }
 }
 
 # Any feature of Skia that requires third-party code should be optional and use this template.