Do not assume SVG 1.1 for `<svg>` in `getExternalSubset`

[carlosame]

In getExternalSubset, a DTD for SVG version 1.1 is supplied to DTD-less <svg> documents. This behaviour was introduced in xml-dtd 3.1. But due to SVG 1.1 being subtly backwards incompatible with 1.0 (e.g. in font-face), this may break applications that process DTD-less SVG 1.0 documents.

getExternalSubset should only support <svg> documents at the entity level, just like the other non-HTML documents.