[SECURITY] Remove version from default User-Agent

TYPO3 does no longer send the concrete TYPO3 version as
part of the default User-Agent header when doing requests.

Resolves: #82072
Releases: master, 8.7, 7.6
Security-Commit: f5558a5d745f2bafe3c27d5621ef1ce26f3989a9
Security-Bulletin: TYPO3-CORE-SA-2017-006
Change-Id: If9d7745d909e93899e2b405e016518a9284a1006
Reviewed-on: https://review.typo3.org/53903


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>