[TASK] English XLIFF files should not contain target element

Pootle uses the English XLIFF file has the template language and as such
it does not make sense to have "en" -> "en" translation files.

English XLIFF files are templates and should contain only a 'source', not
a 'target' element.

This commit contains:

- Remove of the target elements
- Remove target-language attribute
- Set the date attribute to the correct format
- Remove approved attribute

Change-Id: I84f4534f5ed595868c88f05816a73464a76f3f25
Resolves: #31035
Relates: #30916
Releases: 4.6
Reviewed-on: http://review.typo3.org/5916
Tested-by: Philipp Gampe
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers