Represent rabbit_binding:deletions() with a map instead of dict

[the-mikedavis]

This is a mainly cosmetic refactor that makes the types of rabbit_binding:deletions() clearer and easier to work with.

The dict:dict() typing of rabbit_binding appears to be a historical artifact. dict has been superseded by maps. Switching to a map makes deletions easier to inspect manually and faster. Though if deletions grow so large that the map representation is important, manipulation of the deletions is unlikely to be expensive compared to any other operations that produced them, so performance is probably irrelevant.

This commit refactors the bottom section of the rabbit_binding module to switch to a map, switch the deletions() type to an opaque, eliminating a TODO created when using Erlang/OTP 17.1, and the deletion value to a record. We eliminate some historical artifacts and "cruft":

• Deletions taking multiple forms needlessly, specifically the shape {X, deleted | not_deleted, Bindings, none} no longer being handled. process_deletions/2 was responsible for creating this shape. Instead we now use a record to clearly define the fields.
• Clauses to catch {error, not_found} are unnecessary after minor refactors of the callers. Removing them makes the type specs cleaner.
• rabbit_binding:process_deletions/1 has no need to update or change the deletions. This function uses maps:foreach/2 instead and returns ok instead of mapped deletions.
• Remove undefined from the typespec of deletions. This value is no longer possible with a refactor to maybe_auto_delete_exchange_in_* functions for Mnesia and Khepri. The value was nonsensical since you cannot delete bindings for an exchange that does not exist.

[michaelklishin]

The elimination of functions such as anything_but/3 indeed makes the code easier to follow.