MNT: Remove deprecated utils.timer module

This is a follow up of #8898. astropy.utils.timer was deprecated in astropy 4.0 release (#9038) and moved to astroquery 0.4.0 (astropy/astroquery#1508). After a reasonable deprecation period (astropy 4.2?), astropy.utils.timer can be removed completely.

Also un-ignore the deprecation warning added to setup.cfg in #7928.

p.s. Don't forget to also remove the static image in docs/_static.