Fix -path directives for Mongolian

+ Remove references to alltenses.
+ Remove references to ../../prelude
+ Add missing reference to ../common in mongolian/SymbolMon.gf

lib/src/mongolian/NumeralTransferMon.gf

@@ -1,4 +1,4 @@
---# -path=.:../abstract:../common:../../prelude
+--# -path=.:../abstract:../common:../prelude
 
 concrete NumeralTransferMon of NumeralTransfer = NumeralMon ** open ResMon, MorphoMon, CatMon in {
 
@@ -73,4 +73,4 @@ lin
     n = Pl
     } ;
 
-}
+}