Revert "Enable -Wextra-semi in iOS builds."

This reverts commit 8605ee1f.

Reason for revert:
Broke chromium build, details in crbug.com/936211

Original change's description:
> Enable -Wextra-semi in iOS builds.
>
> Bug: 926235
> Change-Id: I1c9f46a30392258e553a51d1033fde4d41e04210
> Reviewed-on: https://chromium-review.googlesource.com/c/1489180
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#635612}

TBR=thakis@chromium.org,sdefresne@chromium.org

Change-Id: I7bd6ed7e9758c6b5b82d2408e4f79fc9d9993bac
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 926235
Reviewed-on: https://chromium-review.googlesource.com/c/1490443
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635762}

build/config/compiler/BUILD.gn

@@ -1591,7 +1591,7 @@ config("chromium_code") {
 
     # TODO(thakis): Enable this for more platforms, https://crbug.com/926235
     has_dchecks = is_debug || dcheck_always_on
-    if (has_dchecks && (is_android || is_ios || is_linux || is_mac || is_win)) {
+    if (has_dchecks && (is_android || is_linux || is_mac || is_win)) {
       cflags += [ "-Wextra-semi" ]
     }
   }