iOS: Disable Goma for iOS GN bots.

The iOS GN bots are the only ones using MB, which seems to enable
Goma by default. Since Goma has started supporting autostart, disable
it for now until a goma start step is added to the bots.

BUG=chromium:617541
TBR=phoglund@webrtc.org

Review URL: https://codereview.webrtc.org/2045583002 .

Cr-Commit-Position: refs/heads/master@{#13049}

webrtc/build/mb_config.pyl

@@ -51,7 +51,7 @@
     },
 
     'debug_bot': {
-      'mixins': ['debug', 'shared', 'goma'],
+      'mixins': ['debug', 'shared'],
     },
 
     'device': {
@@ -61,13 +61,6 @@
 
     'gn': {'type': 'gn'},
 
-    'goma': {
-      # The MB code will properly escape goma_dir if necessary in the GYP
-      # code path; the GN code path needs no escaping.
-      'gn_args': 'use_goma=true goma_dir="$(goma_dir)"',
-      'gyp_defines': 'use_goma=1 gomadir=$(goma_dir)',
-    },
-
     'gyp': {'type': 'gyp'},
 
     'release': {
@@ -75,7 +68,7 @@
     },
 
     'release_bot': {
-      'mixins': ['release', 'static', 'goma'],
+      'mixins': ['release', 'static'],
     },
 
     'shared': {