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

(backported from commit d9f4426c)
[tyhicks: Minor context change]
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>

arch/x86/include/asm/nospec-branch.h

@@ -191,7 +191,6 @@ enum spectre_v2_mitigation {
 	SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
 	SPECTRE_V2_RETPOLINE_GENERIC,
 	SPECTRE_V2_RETPOLINE_AMD,
-	SPECTRE_V2_IBRS,
 };
 
 /* The Intel SPEC CTRL MSR base value cache */