[TASK] XLF core locallang files are still referenced with .php/.xml

The core still access xliff language files as .php or .xml,
although the files themselves are converted to .xlf since quite
some time. This could cause confusion.

This is a cleanup taken all .xlf files in the EXT:lang/.xlf
directory and run via a sed script (thanks to Stucki)

Resolves: #46940
Releases: 6.1
Change-Id: I427776b176ba2c23678689029eaf388dff90d0fa
Reviewed-on: https://review.typo3.org/19579
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn