Remove very old comment referring to a dedup cache

The cache used to help keep binary references around
for fan-out cases, was introduced in 2009 and removed
in 2011. It's no longer relevant...

Author: Loïc Hoguin

deps/rabbit/src/rabbit_msg_store.erl

@@ -242,17 +242,6 @@
 %% function must be provided with a generator that produces ref count
 %% deltas for all recovered messages. This is only used on startup
 %% when the shutdown was non-clean.
-%%
-
-%% @todo Heh? Is that something real? Double check if that actually exists/existed and whether it matters.
-%% Read messages with a reference count greater than one are entered
-%% into a message cache. The purpose of the cache is not especially
-%% performance, though it can help there too, but prevention of memory
-%% explosion. It ensures that as messages with a high reference count
-%% are read from several processes they are read back as the same
-%% binary object rather than multiples of identical binary
-%% objects.
-
 %%
 %% Reads are always performed directly by clients without calling the
 %% server. This is safe because multiple file handles can be used to