[mugenizer] cleantags before removing lead-ins

remove lead-ins correctly for lines that start like this:
	{\fad(300,200)}{\k100}{\k10}

karaoke-adjust-1sec.lua

@@ -116,6 +116,8 @@ function mugenizer(subs)
                 first = i
             end
 
+            line.text = cleantags(line.text)
+
             while hasleadin(line) do
                 if aegisub.progress.is_cancelled() then return end
                 line = removeleadin(line)