mbsync: remove unneeded extraConfig.channel

This was originally placed here, seemingly, just to get this module
working. However, this field is actually more confusing now that a
separate per-channel configuration option for extra configurations has
been made available.

modules/programs/mbsync-accounts.nix

@@ -174,20 +174,6 @@ in {
       '';
     };
 
-    extraConfig.channel = mkOption {
-      type = extraConfigType;
-      default = { };
-      example = literalExample ''
-        {
-          MaxMessages = 10000;
-          MaxSize = "1m";
-        };
-      '';
-      description = ''
-        Per channel extra configuration.
-      '';
-    };
-
     extraConfig.local = mkOption {
       type = extraConfigType;
       default = { };