x86/CET: Remove the -mcet command-lint option

With revision 259496:

commit b1384095a7c1d06a44b70853372ebe037b2f7867
Author: hjl <hjl@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Thu Apr 19 15:15:04 2018 +0000

    x86: Enable -fcf-protection with multi-byte NOPs

-fcf-protection no longer depens on -mcet and with revision 259522:

commit d59cfa9a4064339cf2bd2da828c4c133f13e57f0
Author: hjl <hjl@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Fri Apr 20 13:30:13 2018 +0000

    Define __CET__ for -fcf-protection and remove -mibt

-mcet becomes an alias for -mshstk.  Since all usages of -mcet and
-mno-cet have either been removed or replaced, we can remove the -mcet
command-lint option.

	PR target/85485
	* common/config/i386/i386-common.c (ix86_handle_option): Don't
	handle OPT_mcet.
	* config/i386/i386.opt (mcet): Removed.
	* doc/install.texi: Remove -mcet documentation.
	* doc/invoke.texi: Likewise.

From-SVN: r259613