ARM: OMAP2+: serial: Remove the un-used slave idle hooks

UART IP idle handling now taken care by runtime pm backend(hwmod) indirectly
and OMAP serial driver is also cleaned up accordingly.

So remove the un-used slave idle platforms hooks now.

Tested-by: Vaibhav Bedia <vaibhav.bedia@ti.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Rajendra nayak <rnayak@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>  # OMAP4/Panda
Signed-off-by: Paul Walmsley <paul@pwsan.com>