[!!!][TASK] Streamline TimeTracker global information

This patch removes unused variables:

$GLOBALS['TYPO3_MISC']['microtime_BE_USER_start']
$GLOBALS['TYPO3_MISC']['microtime_BE_USER_end']
$GLOBALS['TYPO3_MISC']['microtime_end']
$GLOBALS['TYPO3_MISC']['microtime_start']

are replaced by properly using
- TimeTracker->start()
- TimeTracker->finish() (new method)

to encapsulate the logic into the main method.

Resolves: #88498
Releases: master
Change-Id: I158e4b3aed002f688f117488cb0300c6523e791f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60769


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>