[TASK] Remove legacy script alt_shortcut.php

Since the backend restructuring for TYPO3 4.2
the alt_shortcut.php file is not used any more.

We can now safely remove it.

Resolves: #55546
Releases: 6.2
Change-Id: I0ee0108c6238a28ad7b2ad0597f421a30eaf118c
Reviewed-on: https://review.typo3.org/27225
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn