Impact: remove unused code

__local_bh_enable has been replaced with _local_bh_enable.
As comments says "it always nests inside local_bh_enable() sections"
has not been valid now. Also there is no reason to use __local_bh_enable
anywhere, so we can remove this useless function.
Signed-off-by: NLiming Wang <liming.wang@windriver.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>