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

These contributes to circular dependency problems in WebRTC
since one have to depend on webrtc.gyp in order to depend on
a target in them.

This reduces the number of cyclic dependencies in WebRTC from 21
to 16.

BUG=webrtc:4243
NOTRY=True
NOPRESUBMIT=True

Review URL: https://codereview.webrtc.org/1839763004

Cr-Commit-Position: refs/heads/master@{#12166}
diff --git a/webrtc/api/api_tests.gyp b/webrtc/api/api_tests.gyp
index bb706dc..b7cbd68 100644
--- a/webrtc/api/api_tests.gyp
+++ b/webrtc/api/api_tests.gyp
@@ -17,7 +17,7 @@
         '<(webrtc_root)/api/api.gyp:libjingle_peerconnection',
         '<(webrtc_root)/base/base_tests.gyp:rtc_base_tests_utils',
         '<(webrtc_root)/common.gyp:webrtc_common',
-        '<(webrtc_root)/webrtc.gyp:rtc_unittest_main',
+        '<(webrtc_root)/media/media.gyp:rtc_unittest_main',
         '<(webrtc_root)/pc/pc.gyp:rtc_pc',
       ],
       'direct_dependent_settings': {