ubsan and valgrind continue to report use after delete of SwSectionFrm

with the same error message listed in the original commit of

commit c3087d96

    avoid crash on layout of ooo77837-1.odt

    by attempting to set the parent as un-joinable for the duration of the
    Cut/Paste in order to ensure it survives the process

which introduced FlowFrmJoinLockGuard

continue to use lighter weight SwFrmDeleteGuard where we can get away with
doing that.

Change-Id: Ifb4c69514d074c776ec036987153043d1f715b95