Remove incomplete and dated 'wgUseETag' experiment

Remove incomplete and dated 'wgUseETag' experiment

Follows-up:
* 8ff36ebd (r9793) Set ETag header on article views.
* 8167b89e (r16205) Add $wgUseETag, false by default (broke Squid; T9098).

ETag is a useful alternative for If-Modified-Since (see T33639, T49529 about
use cases and bugs it may fix). However the current incomplete implementation
isn't useful.

It's a feature flag that has been disabled since 2006. Enabling it stores
ParserCache::getETag in OutputPage and sends it out as a response header.
It's ignored on incoming requests. We don't check If-None-Match for
'304 Not Modified' anywhere.

ResourceLoader has a working implementation that could be leveraged.

To be re-evaluated in the future. ParserCache::getETag() could use
improvement as well.

Change-Id: I37fd9d0ce1ce9bffb79243c6ae340403fdfd769f