Remove entry for RPC quota from hbase:quota when RPC quota is removed.

hbase(main):005:0> create 't2','cf1'
Created table t2
Took 0.8000 seconds
=> Hbase::Table - t2
hbase(main):006:0> set_quota TYPE => THROTTLE, TABLE => 't2', LIMIT => '10M/sec'
Took 0.1024 seconds
hbase(main):007:0> list_quotas
OWNER                          QUOTAS
 TABLE => t2                   TYPE => THROTTLE, THROTTLE_TYPE => REQUEST_SIZE, LIMIT => 10M/sec, SCOPE => MACHINE
1 row(s)
Took 0.0622 seconds
hbase(main):008:0> scan 'hbase:quota'
ROW                            COLUMN+CELL
 t.t2                          column=q:s, timestamp=1531513014463, value=PBUF\x12\x0B\x12\x09\x08\x04\x10\x80\x80\x80
                               \x05 \x02
1 row(s)
Took 0.0453 seconds
hbase(main):009:0> set_quota TYPE => THROTTLE, TABLE => 't2', LIMIT => 'NONE'
Took 0.0097 seconds
hbase(main):010:0> list_quotas
OWNER                          QUOTAS
0 row(s)
Took 0.0338 seconds
hbase(main):011:0> scan 'hbase:quota'
ROW                            COLUMN+CELL
 t.t2                          column=q:s, timestamp=1531513039505, value=PBUF\x12\x00
1 row(s)
Took 0.0066 seconds