This is part of the project of converting MVert into float3.
(more details in T93602), The pbvh update flag is removed and
replaced with a bitmap stored in the PBVH structure. This
patch is similar to D13878. This is mainly setup for an eventual
performance improvement by removing the extra data from
mesh vertices, but if it's consistent with testing in the other patch
doing the same thing for another "temp tag", then it may actually
increase the speed of sculpt code slightly, since less memory needs
to be loaded when checking/changing the flags.

One thing I'm not totally confident about is removing the update flag
from SCULPT_dynamic_topology_disable_ex. I did that because
ss->pbvh was null at that point, which suggests to me that
everything will be rebuilt anyway when the PBVH is recreated.
Disabling dynamic topology seems to work as expected.