Remove unused GetSingleSignatureRecordProvider

As single signatures are retrieved alongside their open message this
provider never found its use, it's been several months so we can safely
delete it.

Author: Alenar

mithril-aggregator/src/database/provider/single_signature/get_single_signature.rs

@@ -1,6 +1,3 @@
-mod get_single_signature;
 mod update_single_signature;
 
-#[allow(unused_imports)]
-pub use get_single_signature::*;
 pub use update_single_signature::*;