build: remove unused variables

detected by clang.

Change-Id: Id9effcc5437870f37fecd33803f7753c6eca53d6
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/361
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Reviewed-by: Mathias Küster <kesmtp@freenet.de>

src/target/stm32_stlink.c

@@ -511,7 +511,6 @@ static int stm32_stlink_step(struct target *target, int current,
 		return ERROR_TARGET_NOT_HALTED;
 	}
 
-	pc = armv7m->arm.pc;
 	if (!current) {
 		buf_set_u32(pc->value, 0, 32, address);
 		pc->dirty = true;