Fix -path directives for Mongolian

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

lib/src/api/SyntaxMon.gf

@@ -1,4 +1,3 @@
---# -path=.:./alltenses:../prelude
 
 instance SyntaxMon of Syntax = 
   ConstructorsMon, CatMon, StructuralMon, CombinatorsMon ;