Remove redundant or unnecessary symbols from `__known_symbols__`.

When creating Python wrapper modules for COM libraries, `comtypes` imports symbols specified in `__known_symbols__` from existing modules instead of generating them in the codebase.

`__known_symbols__` was introduced in #360 to prevent unintentionally replacing symbols in Python wrapper modules when modifying the codebase to define new symbols.

However, `__known_symbols__` also includes integer constants and Python utility symbols specific to the `comtypes` project. Their inclusion makes `__known_symbols__` harder to read.

Moreover, if a constant with the same value already exists in the COM library, replacing it with an import is of little significance.

I propose removing unnecessary symbols that are not interfaces, structures, or unions.