Deprecate AMP_QUERY_VAR constant and amp_query_var filter

[westonruter]

Currently one is able to override the amp query var and endpoint to be something totally different:

https://github.com/Automattic/amp-wp/blob/c701b0b8614e4b454bf620950018d0f143796d3c/amp.php#L80-L88

This ability complicates some things because we have to wait until the theme has fully loaded before we can start looking for whether it is an AMP request or not.

I want to find out if this ability is actually being used. And if so, why. If it is not, then we should remove it.