fix: remove reference to undefined need 'tool_req__docs_metamodel'

The needextend directive was adding ':+satisfies: tool_req__docs_metamodel'
to all needs with 'metamodel.yaml' in their source_code_link. However,
the need 'tool_req__docs_metamodel' is not defined anywhere, causing 39
'[needs.link_outgoing]' Sphinx warnings which are fatal under -W.

Author: Arnaud Riess

docs/internals/requirements/requirements.rst

@@ -1171,5 +1171,4 @@ Overview of Tool to Process Requirements
   :status: valid
 
 .. needextend:: source_code_link is not None and "metamodel.yaml" in source_code_link
-  :+satisfies: tool_req__docs_metamodel
   :+tags: config