Remove BORINGSSL_CLANG_SUPPORTS_DOT_ARCH.

This is no longer necessary as of
https://boringssl-review.googlesource.com/24124.

Change-Id: I704668dd9bf9c1a08c7574c5b95b090485a7ade5
Reviewed-on: https://chromium-review.googlesource.com/924547Reviewed-by: Steven Valdez <svaldez@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537843}

third_party/boringssl/BUILD.gn

@@ -62,7 +62,6 @@ if (is_win && !is_msan) {
   source_set("boringssl_asm") {
     visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
-    defines = [ "BORINGSSL_CLANG_SUPPORTS_DOT_ARCH" ]
     sources = []
     asmflags = []
     include_dirs = [ "src/include" ]