drivers/cmsis_dap: drop unused variable

The variable 'mode' was introduced in
commit 4dc8cd201c ("cmsis-dap: add initial cmsis-dap support")
but never has been used (just cleared)

Change-Id: Ia741d181ee8006bd0d872f3358a57e045235741a
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7965
Reviewed-by: zapb <dev@zapb.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

src/jtag/drivers/cmsis_dap.h

@@ -52,7 +52,7 @@ struct cmsis_dap {
 	unsigned int pending_fifo_block_count;
 
 	uint16_t caps;
-	uint8_t mode;
+
 	uint32_t swo_buf_sz;
 	bool trace_enabled;
 };