hpet: remove hpets::hp_clocksource

Commit cf8e8658

 (arch: Remove Itanium (IA-64) architecture) removed
the last user of hpets::hp_clocksource. Drop the member.

Found by https://github.com/jirislaby/clang-struct.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240213111527.25218-1-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>