tty: remove platform_sysrq_reset_seq

The platform_sysrq_reset_seq code was intended as a way for an embedded
platform to provide its own sysrq sequence at compile time. After over
two years, nobody has started using it in an upstream kernel, and
the platforms that were interested in it have moved on to devicetree,
which can be used to configure the sequence without requiring kernel
changes. The method is also incompatible with the way that most
architectures build support for multiple platforms into a single
kernel.

Now the code is producing warnings when built with gcc-5.1:

drivers/tty/sysrq.c: In function 'sysrq_init':
drivers/tty/sysrq.c:959:33: warning: array subscript is above array bounds [-Warray-bounds]
   key = platform_sysrq_reset_seq[i];

We could fix this, but it seems unlikely that it will ever be used,
so let's just remove the code instead. We still have the option to
pass the sequence either in DT, using the kernel command line,
or using the /sys/module/sysrq/parameters/reset_seq file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 154b7a48 ("Input: sysrq - allow specifying alternate reset sequence")
----
v2: moved sysrq_reset_downtime_ms variable to avoid introducing a compile
    warning when CONFIG_INPUT is disabled
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>