edit_post_link() performs redundant permissions checking

[coffee2code]

edit_post_link() duplicates permission checks that are already (and always) performed in get_edit_post_link().

edit_post_link() has checks to ensure current_user_can('edit_post', $post->ID) (and similar if it's a page). If the user can, then get_edit_post_link() is called, which repeats those checks.

The attached patch has edit_post_link() defer to get_edit_post_link() for the check. This eliminates code duplication and redundant processing, centralizes this permission checking, and removes lines of code; wins all around.

Patch is also compatible with changes I suggested in #9898.

[coffee2code]

Patch as mentioned in original submission.

[ryan]

(In [11426]) eliminate duplicate cap checks. Props coffee2code. fixes #9899