Revert "Compile Skia .mm files with C++17 support"

This reverts commit afc005c8.

Reason for revert: prefer https://chromium-review.googlesource.com/c/chromium/src/+/1937657

Original change's description:
> Compile Skia .mm files with C++17 support
> 
> Change-Id: Ied5a58752c15cee95ba053e39e1c31e06d09f361
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937649
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>
> Cr-Commit-Position: refs/heads/master@{#719342}

TBR=bsalomon@google.com,brianosman@google.com

Change-Id: Iab3e82c4e3df4c4f52767a983cce234c794217fb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937970Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#719346}

skia/BUILD.gn

@@ -199,9 +199,6 @@ config("skia_library_config") {
   } else {
     cflags_cc = [ "-std=c++17" ]
   }
-  if (is_mac) {
-    cflags_objcc = [ "-std=c++17" ]
-  }
 }
 
 source_set("skcms") {
@@ -811,7 +808,6 @@ if (is_mac) {
     cflags_objcc = [
       "-Wno-unguarded-availability",
       "-fobjc-arc",
-      "-std=c++17",
     ]
     deps = [
       "//gpu/config:workaround_list",