Cleanup GACSP code

Problem/Motivation

With ga updating support to ^9 || ^10, it no longer has any overlapping supported core version with gacsp (which never supported ^9). There are only a small number of sites continuing to report usage of gacsp, despite its releases being marked as no longer supported since Drupal 8.9 support ended.

Proposed resolution

Remove status message, hook_page_attachments behaviour, etc, that checks for presence of gacsp