[FEATURE] Auto-detect charset conversion

The CharsetConverter previously had to be configured
to detect if mbstring or iconv or TYPO3's homemade code
via the $TYPO3_CONF_VARS options.

Since CharsetConverter is now a singleton, the check
which conversion strategy should be chosen is now done
during runtime once every request, where mbstring
takes precedence over iconv.

The feature detection in the Install Tool is removed.

Resolves: #72337
Releases: master
Change-Id: Iaa34ed9db2faec0eda9606a399541e564499baa3
Reviewed-on: https://review.typo3.org/45663


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>