commit 9f01951b also did the extra
Remove/Add for Draw fly objects, and it turns out that that's actually
wrong because SwTextFlyCnt::SetAnchor() will set the anchor without
locking anything if it's a Draw object.  Replace it with a different
hack in SetAnchor() that applies only if it calls LockModify().

Thanks to Varun Dhall for creating a reproducer document.

Not sure if the LockModify() could be replaced completely, perhaps it's
just an optimization to avoid re-creating layout frames for the fly.

(cherry picked from commit fae87e03)

Conflicts:
	sw/source/core/txtnode/atrflyin.cxx

Change-Id: Ib3236f289c2c4202d48ac378a53ce02130d4ce2c