Remove x11 from libjingle_media

This generates incorrect -lX11 with use_x11=0 in our other build system,
which causes the standalone libjingle_media target to not build.
This patch should fix that. I could remove -lX11 completely, and
libjingle still links fine. So does Chrome if I do the corresponding
change there, so I think this change is safe to make.

BUG=None

Review URL: https://codereview.webrtc.org/1306243013

Cr-Commit-Position: refs/heads/master@{#9869}

talk/libjingle.gyp

@@ -563,8 +563,6 @@
           ],
           'libraries': [
             '-lrt',
-            '-lXext',
-            '-lX11',
           ],
         }],
         ['OS=="win"', {