Remove max_in_memory_length/bytes from QQ config type

Also remove a resolved TODO about conversion for the `last_checkpoint`
field.

Author: the-mikedavis

deps/rabbit/src/rabbit_fifo.erl

@@ -917,7 +917,6 @@ which_module(4) -> ?MODULE.
                gc = #aux_gc{} :: #aux_gc{},
                tick_pid :: undefined | pid(),
                cache = #{} :: map(),
-               %% TODO: we need a state conversion for this
                last_checkpoint :: #checkpoint{}}).
 
 init_aux(Name) when is_atom(Name) ->

deps/rabbit/src/rabbit_fifo.hrl

@@ -220,8 +220,6 @@
                     checkpoint_max_indexes => non_neg_integer(),
                     max_length => non_neg_integer(),
                     max_bytes => non_neg_integer(),
-                    max_in_memory_length => non_neg_integer(),
-                    max_in_memory_bytes => non_neg_integer(),
                     overflow_strategy => drop_head | reject_publish,
                     single_active_consumer_on => boolean(),
                     delivery_limit => non_neg_integer(),