server: remove remaining crust from dropped eCos code

Commit 39650e2273 ("ecosboard: delete bit-rotted eCos code") has
removed eCos code but has left some empty function that was used
during non-eCos build to replace eCos mutex.

Drop the functions and the file that contain them.

Change-Id: I31bc0237ea699c11bd70921660f960ee406ffa80
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6835
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/server/server.c

@@ -487,10 +487,8 @@ int server_loop(struct command_context *command_context)
 				timeout_ms = polling_period;
 			tv.tv_usec = timeout_ms * 1000;
 			/* Only while we're sleeping we'll let others run */
-			openocd_sleep_prelude();
 			kept_alive();
 			retval = socket_select(fd_max + 1, &read_fds, NULL, NULL, &tv);
-			openocd_sleep_postlude();
 		}
 
 		if (retval == -1) {