New revisions code should use wp_revisions_enabled() rather than referencing the WP_POST_REVISIONS constant

[ethitter]

With the changes made as part of #22289, the WP_POST_REVISIONS constant is no longer the definitive indicator that revisions are enabled, wp_revisions_enabled() is.

r23769 introduced a reference to the constant in wp-admin/revision.php as wp_revisions_enabled() didn't exist yet. The attached patch replaces the reference to the constant and the check that the current post type supports revisions, as wp_revisions_enabled() handles both checks.

[nacin]

In 24425:

Use wp_revisions_enabled(). props ethitter. fixes #24525.