Restore ability to customize 'amp' query var when theme support added

[westonruter]

In #1148 (PR #1194) it was proposed that the ability to customize the amp query var should be eliminated. This was to facilitate themes and plugins to be able to look at the query var early before the parse_query action to determine if it is going to be an AMP response. However, this had to be undone in #1235 because it is not possible to know whether AMP should be available before parse_query because we need the query vars and queried object in order to determine whether the given request is among the supported post types, supported templates, or if AMP has been disabled for the entire post itself. So the reason for this code is now obsolete.

However, there is also a bug that this code introduced. For example, consider a plugin that customizes the slug to lite:

add_filter( 'amp_query_var', function() {
	return 'lite';
} );

On such a site in classic mode, the URL to a post would be https://example.com/2018/09/14/foo/lite/

If, however, a AMP were switched over to the new paired mode at present, then the query var is forcibly made amp and the above URL would no longer return an AMP version at all but rather the non-AMP version (since the endpoint then would be ignored). No redirect from /lite/ to ?amp would be done. This could be problematic for indexes that have cached the URLs to the AMP version but the AMP version is lost from the URL.

So this PR removes the unnecessary code and also fixes the above bug.