UBUNTU: SAUCE: remove ibrs_dump sysctl interface

UBUNTU: SAUCE: remove ibrs_dump sysctl interface

BugLink: http://bugs.launchpad.net/bugs/1755627
The ibrs_dump sysctl interface landed in the Ubuntu backport
df043b74ba71 ("x86/spec_ctrl: Add sysctl knobs to enable/disable
SPEC_CTRL feature") but nothing like it reached mainline.

This debug interface spams dmesg with many lines of output each time
/proc/sys/kernel/ibrs_dump is accessed (notably, every run of 'sysctl -a')
The interface returns only a dummy sysctl value; it has no other purpose
aside from generating dmesg output.

Remove the interface to squelch the excessive dmesg logging by 'sysctl -a'.

Fixes: df043b74ba71 ("x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature")
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Leann Ogasawara <leann.ogasawara@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>