hash: remove dead verify_cell_range and debug scaffolding

hash: remove dead verify_cell_range and debug scaffolding

verify_cell_range and dump_bucket_locked guarded against hash table
corruption from bees bugs fixed long ago.  The periodic survey was the last
live caller of verify_cell_range; it dropped that call when it switched to
summing per-extent occupancy fragments, leaving the function referenced only
from #if 0 blocks.

Delete verify_cell_range, dump_bucket_locked, and the VERIFY_CLEARS_BUGS
flag, along with the three disabled call sites in erase_hash_addr,
push_front_hash_addr and push_random_hash_addr.  The push_random case_cond
variable existed solely to label cases in that disabled diagnostic, so it
goes too; the goto targets that drove the control flow remain.

Assisted-by: Claude-Code:claude-opus-4-8
Signed-off-by: Zygo Blaxell <bees@furryterror.org>