Removing redundant X-Cache header handling.

X-Cache handling is not needed because the pagespeed server never gets to see
a request with X-Cache header added by its downstream cache. X-Cache headers
are only to be added in responses served out of the downstream cache for debugging
purposes only. This piece of deleted code was also conflicting with X-Cache
headers present on upstream caches (wrt pagespeed) and hence not optimizing responses
that happened to carry this header.

(More details are present in https://github.com/pagespeed/ngx_pagespeed/issues/488)