mwifiex: remove low priority scan handling

We have a logic in driver to delay or abort low priority scan
to serve Tx traffic effectively. With scan channel
gap support added, firmware now allows driver to send Tx data while
scan operation is in progress. Hence low priority scan handling
in driver is not required now. This patch removes related code.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>