row_cache: Drop unnecessary assignment to _lower_bound on exception
row_cache: Drop unnecessary assignment to _lower_bound on exception

We no longer drain cached tombstones since commit
41ede08a1dd269eff1c2e34c667347fcc448515a, so this adjustment of
lower_bound is not needed.

Message-Id: <9979100e185e17ae5399676c6f328b543638759b@scylladb.com>

duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla