Update internal.*.webrtc iOS bots iOS version, remove redundant config file

This is a reland of 9a27c2fc7a2eb068eb06fad2e991039e2fceb960

Original change's description:
> Make internal.*.webrtc iOS bots dimensions depend on bot_id and pool only
>
> Bug: webrtc:10047
> Change-Id: I48bcc15721859062c73c7e5fbae7a2fd65713adc
> Reviewed-on: https://webrtc-review.googlesource.com/c/120615
> Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
> Commit-Queue: Artem Titarenko <artit@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#26485}

No-Try: true
Bug: webrtc:10047
Change-Id: Ib9f765439dc58d4e63a201d746129f99e8271175
Reviewed-on: https://webrtc-review.googlesource.com/c/120803
Commit-Queue: Artem Titarenko <artit@webrtc.org>
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26508}

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

@@ -20,7 +20,9 @@
     {
       "include": "perf_tests.json",
       "device type": "iPhone 7",
-      "os": "11.0.3",
+      "os": "12.1.3",
+      "pool": "WebRTC",
+      "bot id": "build15-a7",
       "max runtime seconds": 7200
     }
   ]

tools_webrtc/ios/internal.tryserver.webrtc/ios_arm64_perf.json

@@ -18,9 +18,11 @@
   "bucket": "chromium-webrtc",
   "tests": [
     {
-      "include": "perf_trybot_tests.json",
+      "include": "perf_tests.json",
       "device type": "iPhone 7",
-      "os": "11.0.3",
+      "os": "12.1.3",
+      "pool": "WebRTC",
+      "bot id": "build16-a7",
       "max runtime seconds": 7200
     }
   ]

tools_webrtc/ios/tests/perf_tests.json

@@ -4,9 +4,7 @@
       "app": "webrtc_perf_tests",
       "test args": [
         "--save_chartjson_result"
-      ],
-      "bot id": "build15-a7",
-      "pool": "WebRTC"
+      ]
     }
   ]
 }

tools_webrtc/ios/tests/perf_trybot_tests.json

@@ -1,12 +0,0 @@
-{
-  "tests": [
-    {
-      "app": "webrtc_perf_tests",
-      "test args": [
-        "--save_chartjson_result"
-      ],
-      "bot id": "build16-a7",
-      "pool": "WebRTC"
-    }
-  ]
-}