Revert "Explicitly set target_subarch for iOS."

This reverts commit f6b7265c6b.

Reason: Did not have the desired effect of resolving
https://webrtc-codereview.appspot.com/50569004/ due to incorrect
local testing. I suspect these lines need to be at another
variables-level.

TBR=tkchin@webrtc.org
BUG=4503

Review URL: https://webrtc-codereview.appspot.com/47989004

Cr-Commit-Position: refs/heads/master@{#8939}

webrtc/build/common.gypi

@@ -147,14 +147,6 @@
       ['OS=="ios"', {
         'build_libjpeg%': 0,
         'enable_protobuf%': 0,
-        'conditions': [
-          ['target_arch=="arm"', {
-            'target_subarch%': 'arm32',
-          }],
-          ['target_arch=="arm64"', {
-            'target_subarch%': 'arm64',
-          }],
-        ],
       }],
       ['target_arch=="arm" or target_arch=="arm64"', {
         'prefer_fixed_point%': 1,