Doctool: Remove version attribute from XML header

We don't use that info for anything, and it generates unnecessary diffs
every time we bump the minor version (and CI failures if we forget to
sync some files from opt-in modules (mono, text_server_fb).

doc/tools/make_rst.py

@@ -607,10 +607,6 @@ def main() -> None:
             continue
         doc = tree.getroot()
 
-        if "version" not in doc.attrib:
-            print_error(f'{cur_file}: "version" attribute missing from "doc".', state)
-            continue
-
         name = doc.attrib["name"]
         if name in classes:
             print_error(f'{cur_file}: Duplicate class "{name}".', state)