C#: Remove some unused legacy relations from the DB scheme

csharp/downgrades/58a680081d22ec56c376539b7cca9521dc38fdbb/upgrade.properties

@@ -0,0 +1,2 @@
+description: Remove unused legacy relations.
+compatibility: backwards

csharp/ql/lib/semmlecode.csharp.dbscheme

@@ -166,67 +166,17 @@ compilation_assembly(
   int assembly: @assembly ref
 )
 
-/*
- * External artifacts
- */
-
-externalDefects(
-  unique int id: @externalDefect,
-  string queryPath: string ref,
-  int location: @location ref,
-  string message: string ref,
-  float severity: float ref);
-
-externalMetrics(
-  unique int id: @externalMetric,
-  string queryPath: string ref,
-  int location: @location ref,
-  float value: float ref);
-
-externalData(
-  int id: @externalDataElement,
-  string path: string ref,
-  int column: int ref,
-  string value: string ref);
-
-snapshotDate(
-  unique date snapshotDate: date ref);
-
 sourceLocationPrefix(
   string prefix: string ref);
 
-/*
- * Duplicate code
- */
-
-duplicateCode(
-  unique int id: @duplication,
-  string relativePath: string ref,
-  int equivClass: int ref);
-
-similarCode(
-  unique int id: @similarity,
-  string relativePath: string ref,
-  int equivClass: int ref);
-
-@duplication_or_similarity = @duplication | @similarity
-
-tokens(
-  int id: @duplication_or_similarity ref,
-  int offset: int ref,
-  int beginLine: int ref,
-  int beginColumn: int ref,
-  int endLine: int ref,
-  int endColumn: int ref);
-
 /*
  * C# dbscheme
  */
 
 /** ELEMENTS **/
 
 @element = @declaration | @stmt | @expr | @modifier | @attribute | @namespace_declaration
-         | @using_directive | @type_parameter_constraints | @external_element
+         | @using_directive | @type_parameter_constraints
          | @xmllocatable | @asp_element | @namespace | @preprocessor_directive;
 
 @declaration = @callable | @generic | @assignable | @namespace;
@@ -239,8 +189,6 @@ tokens(
 
 @assignable_with_accessors = @property | @indexer;
 
-@external_element = @externalMetric | @externalDefect | @externalDataElement;
-
 @attributable = @assembly | @field | @parameter | @operator | @method | @constructor
               | @destructor | @callable_accessor | @value_or_ref_type | @declaration_with_accessors
               | @local_function;