tools/clean_svg: do not remove an elem while iterating on a list

Doing a simple is undefined:
    for e in ls:
      if e.matches(foo):
        ls.remove(e)

Basically we were skipping the next entry, still leaving some metadata
in the cleaned up svg file.

tools/clean_svg.py

@@ -84,7 +84,7 @@ def round_if_number(value):
 def remove_non_svg_nodes_and_strip_namespace(root):
     if root.tag.startswith(BRACKETS_NAMESPACE):
         root.tag = root.tag[len(BRACKETS_NAMESPACE) :]
-    for elem in root:
+    for elem in list(root):
         if (
             not elem.tag.startswith(BRACKETS_NAMESPACE)
             or elem.tag == BRACKETS_NAMESPACE + "metadata"