Remove {media,p2p,pc,xmllite,xmpp}_tests.gypi files.

webrtc/pc/rtc_pc_unittests.isolate

 # Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 {
   'conditions': [
     ['OS=="linux" or OS=="mac" or OS=="win"', {
       'variables': {
         'command': [
           '<(PRODUCT_DIR)/rtc_pc_unittests<(EXECUTABLE_SUFFIX)',
         ],
         'files': [
           '<(PRODUCT_DIR)/rtc_pc_unittests<(EXECUTABLE_SUFFIX)',
         ],
       },
     }],
   ],
 }