rtw88: Remove duplicate include of coex.h

In commit fb8517f4 ("rtw88: 8822c: add CFO tracking"),
"coex.h" was added here which caused the duplicate include.
Remove the later duplicate include.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210430024951.33406-1-wanjiabing@vivo.com