[flang] Remove extraneous Pre/Post for ProcedureDeclarationStmt

clang give a compilation error on resolve-names.cc because there are
two overloadings of Pre(ProcedureDeclarationStmt) available in
ResolveNamesVisitor. One is defined in DeclTypeSpecVisitor and the other
in DeclarationVisitor. They are both brought in to ResolveNamesVisitor
via `using` statements.

The one in DeclarationVisitor is the one that is supposed to be called.
The other should have been removed when this one was added. This is the
one that gcc chooses to call, so this doesn't change any behavior.

The same applies to the Post method as well.

Original-commit: flang-compiler/f18@872f8faf96164a01f18100fc1942f68523be6d76
Reviewed-on: https://github.com/flang-compiler/f18/pull/100