shadow-dom: Drop section numbers from directory names.

This change is based on the discussion at
<https://github.com/w3c/web-platform-tests/pull/130>.

There is no change in test contents.

---

FWIW, I used the following one liner (it's wrapped for readability) at the shadow-dom directory:
```sh
$ for D in $(find . -type d | sort | tac); \
> do if [ -n "$(grep -E '[0-9][0-9]-' <(basename $D))" ]; \
> then git mv $D \
> $(dirname $D)/$(sed 's/[0-9][0-9]-//' <(basename $D)); \
> fi; done
```