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/CombinatorsMon.gf

@@ -1,4 +1,3 @@
---# -path=.:alltenses:prelude
 
 resource CombinatorsMon = Combinators with 
   (Cat = CatMon),

lib/src/api/ConstructorsMon.gf

@@ -1,3 +1,2 @@
---# -path=.:alltenses:prelude
 
 resource ConstructorsMon = Constructors with (Grammar = GrammarMon) ;

lib/src/api/SyntaxMon.gf

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