Remove redundant or unnecessary symbols from `__known_symbols__`.

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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.