[ios] Remove override of ios_use_goma_rbe gn variable

The gn variable ios_use_goma_rbe is ignored since the CL
https://crrev.com/c/2555117 landed, so stop overriding
it on the bots (it is now always enabled which is what
the bots want).

Bug: none
Change-Id: Iaa085dd1fd0559a41372744ed4c4491c4b5d9908
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2558218Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71403}

infra/mb/mb_config.pyl

@@ -696,7 +696,7 @@
     },
 
     'ios_simulator': {
-      'gn_args': 'target_cpu="x64" target_os="ios" ios_use_goma_rbe=true',
+      'gn_args': 'target_cpu="x64" target_os="ios"',
     },
 
     'lsan': {