chore: remove default database backend

has been sqlite for far too long, and having a default for this is just asking for trouble

src/config/mod.rs

@@ -21,7 +21,6 @@ pub struct Config {
     pub tls: Option<TlsConfig>,
 
     pub server_name: OwnedServerName,
-    #[serde(default = "default_database_backend")]
     pub database_backend: String,
     pub database_path: String,
     #[serde(default = "default_db_cache_capacity_mb")]
@@ -223,10 +222,6 @@ fn default_port() -> u16 {
     8000
 }
 
-fn default_database_backend() -> String {
-    "sqlite".to_owned()
-}
-
 fn default_db_cache_capacity_mb() -> f64 {
     300.0
 }