kernel_lockdown.7: Remove description of lifting via SysRq (not upstream)

kernel_lockdown.7: Remove description of lifting via SysRq (not upstream)

The patch that implemented lockdown lifting via SysRq ended up
getting dropped[*] before the feature was merged upstream. Having
the feature documented but unsupported has caused some confusion
for our users.

[*] http://archive.lwn.net:8080/linux-kernel/CACdnJuuxAM06TcnczOA6NwxhnmQUeqqm3Ma8btukZpuCS+dOqg@mail.gmail.com/

Signed-off-by: dann frazier <dann.frazier@canonical.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Pedro Principeza <pedro.principeza@canonical.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Kyle McMartin <kyle@redhat.com>
Cc: Matthew Garrett <mjg59@google.com>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>