Removed build dependency on er_tables_xor.h, since it has been deleted

As part of https://webrtc-codereview.appspot.com/45899004/ the file er_tables_xor.h was removed, but not its dependencies in .gn and .gypi.

BUG=N/A
TBR=pbos

Review URL: https://webrtc-codereview.appspot.com/48889004

Cr-Commit-Position: refs/heads/master@{#8944}

webrtc/modules/video_coding/video_coding.gypi

@@ -30,7 +30,6 @@
         'main/source/content_metrics_processing.h',
         'main/source/decoding_state.h',
         'main/source/encoded_frame.h',
-        'main/source/er_tables_xor.h',
         'main/source/fec_tables_xor.h',
         'main/source/frame_buffer.h',
         'main/source/generic_decoder.h',