Remove nonexistant test.

I did some archaelogy here and found that the binary was (incorrectly?)
added here: https://codereview.webrtc.org/1903663002/. I can't find any
trace that a rtc_sdk_peerconnection_objc_tests has ever existed, or at
least that it doesn't exist now.

Removing it seems the right thing to do. However, I also see
rtc_sdk_peerconnection_objc target is folded into rtc_unittests in
webrtc_tests.gyp, but that target isn't listed in common_tests.json.
Should it be?

BUG=623500
TBR=tkchin@webrtc.org

Review-Url: https://codereview.webrtc.org/2121963002
Cr-Commit-Position: refs/heads/master@{#13392}

webrtc/build/ios/tests/common_tests.json

@@ -12,9 +12,6 @@
     {
       "app": "modules_tests"
     },
-    {
-      "app": "rtc_sdk_peerconnection_objc_tests"
-    },
     {
       "app": "rtc_pc_unittests"
     },