enable unused_self lint

Functions using `services()` are allowed to pointlessly take `self`
because the existence of `services()` is a crime and the solution is
making the types store references to their dependencies and then going
through `self`, so just allowing the lint saves us from modifying some
code only to switch it back later. Much later. Getting rid of
`services()` will probably be an ordeal.

Cargo.toml

@@ -91,6 +91,7 @@ unnested_or_patterns = "warn"
 unreadable_literal = "warn"
 unseparated_literal_suffix = "warn"
 unused_async = "warn"
+unused_self = "warn"
 verbose_file_reads = "warn"
 wildcard_dependencies = "warn"