[TASK] Remove unused workspace_preview DB field

The unused BE_USER method setWorkspacePreview()
updates a DB field called "workspace_preview_id" which just
does not exist at all. It would lead to an SQL error if called.

However, since this method is never called, nor evaluated since TYPO3 6.2,
all functionality should be removed. Previewing a workspace is handled
via `be_users.workspace_id` anyway.

Resolves: #84021
Releases: master
Change-Id: I706e893aef1faaccf6e43f0e66ac28bff79190d8
Reviewed-on: https://review.typo3.org/55871


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>