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}
2 files changed