build: remove unused once_cell dependency

Author: link2xt

Cargo.toml

@@ -36,7 +36,6 @@ futures = "0.3.15"
 imap-proto = "0.16.4"
 log = "0.4.8"
 nom = "7.0"
-once_cell = "1.8.0"
 pin-project = "1"
 pin-utils = "0.1.0-alpha.4"
 self_cell = "1.0.1"