rtt: drop unused function rtt_started()

The function is not used.
Drop it!

Change-Id: I176c9d6ba077e36b762c14f9b877d5152992763c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8545
Tested-by: jenkins
Reviewed-by: zapb <dev@zapb.de>

src/rtt/rtt.c

@@ -297,11 +297,6 @@ int rtt_write_channel(unsigned int channel_index, const uint8_t *buffer,
 		length, NULL);
 }
 
-bool rtt_started(void)
-{
-	return rtt.started;
-}
-
 bool rtt_configured(void)
 {
 	return rtt.configured;