Remove empty 'required' section from schema.md (#8900)

There is a typo in the schema.md file in documentation. An empty 'required' section was added right before the 'required' item in the middle of the attributes list. It even appears in the table of content, which might confuse readers, making it look like following attributes might be required while they are not. You can notice the issue there: https://flatbuffers.dev/schema/#attributes.

I did check that `mkdocs serve -f mkdocs.yml` does produce the expected output (the same attributes list without that extra empty `required` section in the middle) with my changes.

docs/source/schema.md

@@ -427,9 +427,6 @@ Current understood attributes:
   it won't be accessible anymore by newer code. Note that if you deprecate a
   field that was previous required, old code may fail to validate new data (when
   using the optional verifier).
-
-### `required`
-
 - `required` (on a non-scalar table field): this field must always be set. By
   default, fields do not need to be present in the binary. This is desirable, as
   it helps with forwards/backwards compatibility, and flexibility of data