Users Guide: Drop dead code for Haddock refs to `parallel`

I noticed while working on !11451 that `@LIBRARY_parallel_UNIT_ID@` was
not substituted. It is dead code -- there is no `parallel-ref` usages
and it doesn't look like there ever was (going back to
3e5d0f18), so let's delete it.

docs/users_guide/conf.py

@@ -277,7 +277,6 @@ def setup(app):
     app.add_role('cabal-ref', haddock_role('Cabal'))
     app.add_role('ghc-compact-ref', haddock_role('ghc-compact'))
     app.add_role('ghc-prim-ref', haddock_role('ghc-prim'))
-    app.add_role('parallel-ref', haddock_role('parallel'))
     app.add_role('array-ref', haddock_role('array'))
 
     app.add_object_type('rts-flag', 'rts-flag',

docs/users_guide/ghc_config.py.in

@@ -23,7 +23,6 @@ lib_versions = {
     'template-haskell': '@LIBRARY_template_haskell_UNIT_ID@',
     'ghc-compact': '@LIBRARY_ghc_compact_UNIT_ID@',
     'ghc': '@LIBRARY_ghc_UNIT_ID@',
-    'parallel': '@LIBRARY_parallel_UNIT_ID@',
     'Cabal': '@LIBRARY_Cabal_UNIT_ID@',
     'array': '@LIBRARY_array_UNIT_ID@',
 }