Remove Powered by WordPress in classic footer template

[westonruter]

Fixes #1749, per #1749 (comment):

I'm wary of providing any option to disable the footer credits. Either the template should have the credits or it should be removed entirely. Decisions not options, and all. Since the AMP plugin isn't part of WordPress.com, I'm inclined to just removed it entirely, and on WordPress.com a mu-plugin can be added which re-adds the footer credits as required according to the policies there.

In the end, the footer credits only applies to the Classic mode. When using Native/Paired modes, the active theme's templates are used along with whatever they put in the footer, so this question becomes irrelevant. In any case, I'll also open a PR to remove the footer credits from the plugin for a future release.

Also adds missing wp_generator() to classic post template.