Drop deprecated sr_session_halt().

This has been deprecated in favor of sr_session_stop() since a while.

None of the current frontends use sr_session_halt() anymore, neither
does libsigrok.

proto.h

@@ -96,7 +96,6 @@ SR_API int sr_session_datafeed_callback_add(sr_datafeed_callback_t cb);
 /* Session control */
 SR_API int sr_session_start(void);
 SR_API int sr_session_run(void);
-SR_API int sr_session_halt(void);
 SR_API int sr_session_stop(void);
 SR_API int sr_session_save(const char *filename, const struct sr_dev_inst *sdi,
 		unsigned char *buf, int unitsize, int units);

session.c

@@ -366,20 +366,6 @@ SR_API int sr_session_run(void)
 	return SR_OK;
 }
 
-/**
- * Halt the current session.
- *
- * This function is deprecated and should not be used in new code, use
- * sr_session_stop() instead. The behaviour of this function is identical to
- * sr_session_stop().
- *
- * @return SR_OK upon success, SR_ERR_BUG if no session exists.
- */
-SR_API int sr_session_halt(void)
-{
-	return sr_session_stop();
-}
-
 /**
  * Stop the current session.
  *