Remove references to HTML Media Element

[Malvoz]

@Malvoz wrote https://github.com/Maps4HTML/MapML/pull/142#discussion_r517677147:

we're currently describing the map element as a media element:

The map element, in conjunction with child layer and optionally area elements, defines a map media element.

That means the element interface for <map> should be "HTMLMapElement : HTMLMediaElement".

But, I am responsible for linking to the "media element" WHATWG definition, which I first should ask, did you intend to define the map element as a "media element" per that definition (with the presumption that the definition would be extended to include map)?

@prushforth wrote https://github.com/Maps4HTML/MapML/pull/142#issuecomment-722019461:

@Malvoz no I think the HTMLMediaElement interface would be incorrect for the map element. The use of the term 'media' here refers to MIME media types, at most, although it's not clear thattjeterm should be use in this context. I will review and reword.