Remove dynamic-topology flag from mesh when entering sculpt mode

This will be enabled if the file was saved with dynamic topology on,
but we don't automatically re-enter dynamic-topology mode when loading
a file so remove the flag.

Fixes bug [#33956]
projects.blender.org/tracker/?func=detail&aid=33956&group_id=9&atid=498

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54064 954f8c5b-7b00-dc11-b283-0030488c597c