[iOS] Remove code coverage config from build/config/ios/BUILD.gn.

This is duplicated to the same config set in build/config/BUILDCONFIG.gn
causing 2 sets of coverage compiler flags in compile command.

Bug: 1137569
Change-Id: Ic4f712ff098e6e8b036576782d5f1a09f693da32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463853Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816623}

build/config/ios/BUILD.gn

@@ -2,7 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/coverage/coverage.gni")
 import("//build/config/ios/ios_sdk.gni")
 import("//build/config/sysroot.gni")
 import("//build/toolchain/toolchain.gni")
@@ -131,10 +130,6 @@ config("runtime_library") {
   asmflags = common_flags
   cflags = common_flags
   ldflags = common_flags
-
-  if (use_clang_coverage) {
-    configs = [ "//build/config/coverage:default_coverage" ]
-  }
 }
 
 config("ios_executable_flags") {