docs: kernel-parameters.txt: Remove nompx

x86/mpx was removed in commit 45fc24e8
("x86/mpx: remove MPX from arch/x86"), this removes the documentation of
parameter nompx.

Fixes: 45fc24e8

 ("x86/mpx: remove MPX from arch/x86")
Signed-off-by: Jimmy Assarsson <jimmyassarsson@gmail.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20200402172614.3020-1-jimmyassarsson@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>