Bazel CI: Let the CI pick the correct Xcode version.

[bazelbuild#1542](bazelbuild/continuous-integration#1542) added some logic to pick the best Xcode version based on the MacOS version (which unfortunately differs between machines).

This should fix the failures related to [bazelbuild#1540](bazelbuild/continuous-integration#1540) in both pre- and post-submit pipelines.

PiperOrigin-RevId: 506279891
Change-Id: I4f22c1bfdf616a2cc4d06a1e6a242556c3863882

.bazelci/postsubmit.yml

@@ -168,7 +168,6 @@ tasks:
       - build
       - test
   macos:
-    xcode_version: "13.0"
     shell_commands:
       - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
         android_ndk_repository/android_ndk_repository/' WORKSPACE
@@ -213,7 +212,6 @@ tasks:
       - build
       - test
   macos_arm64:
-    xcode_version: "13.0"
     shell_commands:
       - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
         android_ndk_repository/android_ndk_repository/' WORKSPACE

.bazelci/presubmit.yml

@@ -158,7 +158,6 @@ tasks:
       # Re-enable once fixed: https://github.com/bazelbuild/bazel/issues/8162
       - "-//src/java_tools/import_deps_checker/..."
   macos:
-    xcode_version: "13.0"
     shards: 5
     shell_commands:
       - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
@@ -203,7 +202,6 @@ tasks:
       # MacOS does not have cgroups so it can't support hardened sandbox
       - "-//src/test/shell/integration:bazel_hardened_sandboxed_worker_test"
   macos_arm64:
-    xcode_version: "13.0"
     shards: 2
     shell_commands:
       - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#