Remove todo

So in 2009 it was written that combining helped performance.
But I doubt we ever get to a scenario where it matters to
reduce the number of file_summary table entries today.
There's been plenty of ets table optimisations, and this
branch reduces the number of fields in entries anyway
and we don't go over the whole table as often as before.

See 30bc61f

Author: Loïc Hoguin

deps/rabbit/src/rabbit_msg_store.erl

@@ -195,7 +195,6 @@
 %% other words, the valid data accounts for less than half the total
 %% size of the file), we start a garbage collection run concurrently,
 %% which will compact the file. This keeps utilisation high.
-%% @todo The comment used to claim file combining helped performance. Double check?
 %%
 %% The discovery is done periodically on files that had messages
 %% acked and removed. We delibirately do this lazily in order to