remove show-config admin room command

Just `cat` the config file. Also this code would be very annoying to
maintain. Getting rid of this also revealed that another config option
is specific to RocksDB, so `cfg`s for that have been added.

src/service/admin.rs

@@ -156,9 +156,6 @@ enum AdminCommand {
         amount: u32,
     },
 
-    /// Show configuration values
-    ShowConfig,
-
     /// Reset user password
     ResetPassword {
         /// Username of the user for whom the password should be reset
@@ -659,13 +656,6 @@ impl Service {
 
                 RoomMessageEventContent::text_plain("Done.")
             }
-            AdminCommand::ShowConfig => {
-                // Construct and send the response
-                RoomMessageEventContent::text_plain(format!(
-                    "{}",
-                    services().globals.config
-                ))
-            }
             AdminCommand::ResetPassword {
                 username,
             } => {