Remove direct dependency on async_trait (#3382)

Not completely sure it's worth it since some other dependencies still
depend on it. Anyway, proc macros are said to be bad for compile times, I just typed out what the proc macro generates and it's only 8 more lines, and we're already doing it this way in e.g. action_by_contact() and collect_texts_recursive() (the latter needs the boxed future both for the trait and for recursion).

Cargo.toml

@@ -25,7 +25,6 @@ async-smtp = { git = "https://github.com/async-email/async-smtp", branch="master
 async-std-resolver = "0.21"
 async-std = { version = "1" }
 async-tar = { version = "0.4", default-features=false }
-async-trait = "0.1"
 backtrace = "0.3"
 base64 = "0.13"
 bitflags = "1.3"