[BUGFIX] TCA definition is not available in upgrade wizards

The global TCA definition is empty in some upgrade wizards.
This was caused by a general clean-up to remove PHP global
statements that did not seem to be required anymore. However,
if PHP files get included inside a method or function accessing
$TCA did not happen in a global context.

Change-Id: I17df6b3caeda0ff2b5a7399c457ccebc57627882
Resolves: #26515
Reviewed-on: http://review.typo3.org/1809
Reviewed-by: Steffen Gebert
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader