[TASK] Remove last bits of forceCharset handling

Core version 4.5 introduced UTF8 only for charset conversion, and
accepted a fallback under special circumstances. The fallback was
removed with 4.7.
This setting is now finally fully removed from the configuration
and not taken into account anymore.

Change-Id: Ic347c3decb985ec63eb499d5d75827fc7f1f7a8a
Resolves: #49336
Related: #49299
Related: #34092
Releases: 6.2
Reviewed-on: https://review.typo3.org/21584
Reviewed-by: Markus Klein
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn