[BUGFIX] Follow up: Fix ext_icon.gif leftovers

The extension icons were replaced by png pendants in commit
b8d9e285, but some classes
or TCAs still require the gif version.

Resolves: #64866
Releases: master
Change-Id: Ide0c6b99381c5a3e071c2c40c4942212d85e7004
Reviewed-on: http://review.typo3.org/36668


Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>