Drop stale describe_synonym comment

Both lines of the comment above PLSQL::Connection#describe_synonym are
no longer true:

- "all_synonyms view is quite slow" — measured against Oracle 23ai with
  a 1500-synonym fixture this method is ~0.43 ms/call, dominated by
  network round-trip rather than the dictionary-view query. The slow
  case the comment was written for (oracle-enhanced #2686 saw 29x on
  private synonyms in a 1000-object schema) doesn't show up at the
  scales ruby-plsql actually exercises.
- "overridden in OCI connection with faster native OCI method" — that
  override was removed in 4d1f656 (2018, "Remove OCI8#describe_synonym
  to follow up rsim#153") because the predecessor commit 1daa6ce already
  deleted its body and made it call super. There is no longer any
  driver-specific override.

The method's name and SQL are self-explanatory; removing both lines
without a replacement keeps the file accurate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

lib/plsql/connection.rb

@@ -179,8 +179,6 @@ def fetch_hash
       end
     end
 
-    # all_synonyms view is quite slow therefore
-    # this implementation is overriden in OCI connection with faster native OCI method
     def describe_synonym(schema_name, synonym_name) # :nodoc:
       select_first(
         "SELECT table_owner, table_name FROM all_synonyms WHERE owner = :owner AND synonym_name = :synonym_name",