[FEATURE] Make CSS for sysext form configurable

Move the default styles of ext:form to TS in
plugin.tx_form._CSS_DEFAULT_STYLE. This way it can be disabled with
TypoScript either via plugin.tx_form._CSS_DEFAULT_STYLE >, and also
respects the config.removeDefaultCss setting.

Change-Id: Ie2b0c397124f06ec32114983e78dd60b4229ce97
Resolves: #32480
Releases: 6.0
Reviewed-on: http://review.typo3.org/11932
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog