Remove SlotNumber type to keep the PR solely focus on BlockNumber

This commit can be reverted to be the starting point of the PR that add
the `SlotNumber`.

Author: Alenar

mithril-common/src/entities/mod.rs

@@ -22,7 +22,6 @@ mod signed_entity_config;
 mod signed_entity_type;
 mod signer;
 mod single_signatures;
-mod slot_number;
 mod snapshot;
 mod time_point;
 mod type_alias;