Removes reference to withdrawn PEP 459

Removes recommendation for local version identifier metadata.
Resolves #1503

Author: onefifth

source/specifications/version-specifiers.rst

@@ -172,9 +172,6 @@ identified by the public version identifier, but contains additional changes
 indexing and hosting upstream projects, it MUST NOT allow the use of local
 version identifiers.
 
-Source distributions using a local version identifier SHOULD provide the
-``python.integrator`` extension metadata (as defined in :pep:`459`).
-
 
 Final releases
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