Remove lvm-thin pool_size config option

Remove lvm-thin pool_size config option

The pool_size config option when using thin LVM
is never actually checked/honored.  It's also unlikely
that users will want to do something other than use the
entire VG for their thin pool.

Rather than have the unused option going forward let's
remove it from the config and just go with the default
behavior of utilizing the entire Cinder VG.

For folks that would like to only use a portion of the
VG for some reason, they can do an lvcreate pool on their
own (just like we have them do for the VG itself) and get
the same flexibility that way.

This is done simply by:
lvcreate -T -L ${size} ${vg-name}/${vg_name}-pool

DocImpact

Change-Id: I3ef2cde433b8dd775b1873a0d6df165dd1945d8c