toollib: remove unneeded call in process_each_pv

toollib: remove unneeded call in process_each_pv

process_each_pv was doing:

1. lvmcache_seed_infos_from_lvmetad()
   sends pv_list request to lvmetad.

2. get_vgnameids()
   sends vg_list request to lvmetad.

3. _get_all_devices()
   first calls lvmcache_seed_infos_from_lvmetad(),
   which is a no-op if it's already been called.

Because get_vgnameids() does not use the information
from lvmcache_seed_infos_from_lvmetad(), it does not
need to be called prior to get_all_devices where
it is actually needed.