acpi: Remove _OSI(Linux) for ThinkPads

AFAICT the only reason to set _OSI(Linux) on ThinkPads is to get
sensible mute button behavior.  Now that the thinkpad_acpi driver
can do this on is own, there is no reason to keep the ACPI
quirk.

Cc: Len Brown <len.brown@intel.com>
Cc: Jerone Young <jerone.young@canonical.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>