[!!!][TASK] Remove unused constant TYPO3_user_agent

The constant TYPO3_user_agent is removed as it is not in use anymore
since TYPO3 v8.

Resolves: #82296
Releases: master
Change-Id: I4860e291c1888251be5c64c2c454022f5d51643b
Reviewed-on: https://review.typo3.org/53908


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>