[BUGFIX] Remove ElementBrowser::isReadOnlyFolder

ElementBrowser::isReadOnlyFolder is not required any more because the
check if the folder is writable has been moved to the methods that
create the file upload and folder creation forms.

The method and the parts where it was used were removed.

Additionally the check if the user is allowed to create folders
by TSConfig was moved to the createFolder method to reduce the amount
of duplicate code.

Resolves: #47648
Releases: 6.2, 6.1, 6.0
Change-Id: Ic6504c8def80012cbe420fc83539cfa859a53c0d
Reviewed-on: https://review.typo3.org/26468
Reviewed-by: Markus Klein
Tested-by: Markus Klein