serial_core: Remove SUPPORT_SYSRQ ifdeffery

No one defines it anymore.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20191213000657.931618-55-dima@arista.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>