x86/speculation: Remove SPECTRE_V2_IBRS in enum spectre_v2_mitigation

x86/speculation: Remove SPECTRE_V2_IBRS in enum spectre_v2_mitigation

BugLink: https://launchpad.net/bugs/1786139
SPECTRE_V2_IBRS in enum spectre_v2_mitigation is never used. Remove it.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: hpa@zytor.com
Cc: dwmw2@amazon.co.uk
Cc: konrad.wilk@oracle.com
Cc: bp@suse.de
Cc: zhong.weidong@zte.com.cn
Link: https://lkml.kernel.org/r/1531872194-39207-1-git-send-email-jiang.biao2@zte.com.cn
(cherry picked from commit d9f4426c73002957be5dd39936f44a09498f7560)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>