[BUGFIX] Fix Extension Download with compressionLevel enabled

To clear output buffers, the utility function t3lib_div::cleanOutputBuffers()
has to be used. Otherwise things break with compressionLevel enabled.

This change replaces the ob_end_clean() call in the Extension download process,
which did not work, as crap is sent to the user agent.

Change-Id: I078bab66c24d0667b3cd8c2397f26569ed2dbf8d
Resolves: #M18029
Also-Applies: TYPO3_4-5
Reviewed-on: http://review.typo3.org/1264
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog