Fix decrypting files with very long paths on windows

Windows doesn't like very long file paths (< ~250 characters).
When decrypting files stored under such a long file path by clicking on it in the windows explorer, kleopatra fails to decrypt it.
When starting the decryption from within kleopatra, decryption works fine.

In the first case, windows passes us a file path starting with "\\?\", which is supposed to remove file length limitations,
but this doesn't work for us for reasons I don't fully know.

Removing the "\\?\" fixes the problem - even for all cases of very long file paths that i could come up with.

• GnuPG-Bug-ID: T7532