Removes some legacy elements that are not part of the model (#158)

* Removes some legacy elements that are not part of the model

These elements were present in a very early version of SSSOM and now serve no purpose

* Update sssom.yaml

src/linkml/sssom.yaml

@@ -389,22 +389,6 @@ slots:
       by tool providing additional informative information.
     slot_uri: rdfs:comment
     range: string
-  metadata_element:
-    description: All legal SSSOM metadata elements are subproperties of this.
-    range: string
-  scope:
-    description: Indicates whether the metadata element has local, global or local-global
-      scope.
-    range: string
-  rdf_example:
-    description: An example value of the a SSSOM element in the TSV file.
-    range: string
-  tsv_example:
-    description: An example value of the a SSSOM element in RDF.
-    range: string
-  equivalent_property:
-    description: 'SSSOM property should be mapped to:'
-    range: string
 classes:
   mapping set:
     description: Represents a set of mappings