cpuidle: remove dead code from cpuidle_enter_state()

cpuidle: remove dead code from cpuidle_enter_state()

Checking for index < 0 is useless because the find_deepest_state()
function never really returns a negative value.

Since this hasn't been reported in over 9 years it's dead code, so
remove it.

Signed-off-by: Dhruva Gole <d-gole@ti.com>
Link: https://patch.msgid.link/20240821114250.1416421-1-d-gole@ti.com
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>