[BUGFIX] Do not reference image in fileadmin

For the search button there is an image as button
defined. The path references a file in the fileadmin
folder.

The Button is changed to a simple submit button
which is a better default solution for the template
file.

Resolves: #60260

Change-Id: Idd4f010e1f35d378d0705961fe47304683d57f63
Reviewed-on: https://review.typo3.org/31672
Reviewed-by: Sascha Egerer <sascha@sascha-egerer.de>
Tested-by: Sascha Egerer <sascha@sascha-egerer.de>