Use "ios" mixin instead of specifying by ourselves.

For mac builders the number of cores changes based on vm or not.
Also, we expect xcode checkout would be kept for the builders.
Having "ios" mixin is convenient to achieve that.

Bug: 968784
Change-Id: I30d8377fcd944dcf134bb00efa98890fd97e5252
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1644593
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666160}

infra/config/cr-buildbucket.cfg

@@ -2784,8 +2784,7 @@ buckets {
     }
     builders {
       name: "ios-device-goma-canary-clobber"
-      dimensions: "os:Mac"
-      dimensions: "cores:4"
+      mixins: "ios"
       mixins: "fyi-ci"
       recipe {
         name: "ios/unified_builder_tester"
@@ -3255,8 +3254,7 @@ buckets {
     }
     builders {
       name: "ios-device-goma-latest-clobber"
-      dimensions: "os:Mac"
-      dimensions: "cores:4"
+      mixins: "ios"
       mixins: "fyi-ci"
       recipe {
         name: "ios/unified_builder_tester"