loongarch: avoid generating extra header files

The list of generated headers is rather outdated, some of these no longer
exist, while others are already listed in include/asm-generic/Kbuild
so there is no need to list them here.

As we start validating the list of headers against the files that exist,
the outdated ones now cause a warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>