Remove "caused by" marker

As we are in debug mode anyways, a developer using this should know by
the trace number that the next line is the cause.

As we use the debug!() macro here, this would be printed on a new line,
which is rather distracting instead of helpful. Therefor this patch
removes this line.

libimagutil/src/trace.rs

@@ -67,7 +67,6 @@ fn count_error_causes(e: &Error) -> u64 {
 fn print_trace_dbg(idx: u64, e: &Error) {
     debug!("ERROR[{:>4}]: {}", idx, e.description());
     if e.cause().is_some() {
-        debug!(" -- caused by:");
         print_trace_dbg(idx + 1, e.cause().unwrap());
     }
 }