core: Remove LdapSeachGroupsByQuery.initFromDb

Removed LdapSeachGroupByQueryCommand.initFromDb, since all it did was
query an LdapGroup and save it into a local variable that was never used
again.

Note that this patch solves a DLS_DEAD_LOCAL_STORE FindBugs warning on
this class.

Change-Id: I581fab9aa581a3a5093f98cc48edeaffc9080c44
Signed-off-by: Allon Mureinik <amureini@redhat.com>