Partial revert of: 'Bump xcode versions for WebRTC bots.'

Partial revert of https://webrtc-review.googlesource.com/c/src/+/97060.
These machines need to be updated to a newer OS version.

Bug: None
Change-Id: Ice30ff9125eb366a6d6f93080ae7d0bceba1fe8b
Reviewed-on: https://webrtc-review.googlesource.com/98400
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24603}

tools_webrtc/ios/internal.client.webrtc/iOS64_Debug.json

@@ -2,7 +2,7 @@
   "comments": [
     "Tests for 64-bit iOS devices."
   ],
-  "xcode build version": "10l232m",
+  "xcode build version": "9C40b",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
     "is_component_build=false",

tools_webrtc/ios/internal.client.webrtc/iOS64_Perf.json

@@ -2,7 +2,7 @@
   "comments": [
     "Tests for 64-bit iOS devices."
   ],
-  "xcode build version": "10l232m",
+  "xcode build version": "9C40b",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
     "is_component_build=false",

tools_webrtc/ios/internal.client.webrtc/iOS64_Release.json

@@ -2,7 +2,7 @@
   "comments": [
     "Tests for 64-bit iOS devices."
   ],
-  "xcode build version": "10l232m",
+  "xcode build version": "9C40b",
   "gn_args": [
     "dcheck_always_on=true",
     "goma_dir=\"$(goma_dir)\"",