remove unused code from wp-admin/plugins.php

[DD32]

Since the Plugin page was redesigned, Theres some useless code in the header to detect the action.

Attached patch strips it out.

(Note, the clear-recent-list branch is needed as that button has to exist within a form which has an action selection too)

[azaozz]

(In [10867]) Remove unused code from wp-admin/plugins.php, props DD32, fixes #9454