ath9k: Remove "shared_chain_lnadiv"

This variable is redundant since we can use
common->bt_ant_diversity to determine if diversity
has to be enabled/disabled.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>