[BUGFIX] Remove postcss config for EXT:frontend

Since EXT:frontend does not have CSS files anymore, the
postcss config for Grunt is not necessary anymore.

Resolves: #85225
Releases: master
Change-Id: Ibdecf246c2841e383d745dfdfbf7f3a7181be19b
Reviewed-on: https://review.typo3.org/57181


Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>