Remove dead timer code in rabbit_fifo_client

[ansd]

Test case resends_lost_command of rabbit_fifo_int_SUITE succeeded without testing what it is supposed to test.
That's because the mock used the wrong arity.
Applying the following diff

-    meck:expect(ra, pipeline_command, fun (_, _, _) -> ok end),
+    meck:expect(ra, pipeline_command, fun (_, _, _, low) -> ok end),

made the test case then fail with:

rabbit_fifo_int_SUITE > tests > resends_lost_command
    #1. {error,
            {{badmatch,
                 {empty,
                     {state,
                         {cfg,
                             [{resends_lost_command,ct_rabbit@VQD7JFK37T}],
                             32,10000},
                         {resends_lost_command,ct_rabbit@VQD7JFK37T},
                         go,3,4,false,#{},
                         #{1 => {undefined,{e,2,msg2,{0,0}}},
                           2 => {undefined,{e,3,msg3,{0,0}}}},
                         #{},undefined,undefined}}},
             [{rabbit_fifo_int_SUITE,resends_lost_command,1,
                  [{file,"rabbit_fifo_int_SUITE.erl"},{line,315}]},
              {test_server,ts_tc,3,[{file,"test_server.erl"},{line,1796}]},
              {test_server,run_test_case_eval1,6,
                  [{file,"test_server.erl"},{line,1305}]},
              {test_server,run_test_case_eval,9,
                  [{file,"test_server.erl"},{line,1237}]}]}}

The timer was never actually scheduled in rabbit_fifo_client. This commit therefore removes all the dead code.

Note that resend_all_pending/1 is nowadays executed upon leader changes. nodeup in the queue proc also causes resend_all_pending/1 to be executed even if the leader didn't change.