Remove js support from QDoc

QDoc has some support for documenting javascript code-bases.

The support mimics the QML support that QDoc provides by exposing javascript counterparts to the available QML commands.

For example, "\qmlmodule" -> "\jsmodule" or "\qmltype" -> "jstype".

The support was bolted on at some point but was never finalized, with all commands currently missing from the documentation.

Internally, the support is implemented as a copy of the QML support.
The internal structures used to represent QML entities are reused with the difference that their "type", an internal enumeration, is changed to a javascript specific one.

Then, branches that dealt with various QML elements were extended to be valid for javascript elements too, as they use the same internal representation.

This support is currently unused in Qt's codebase, is not public and is not expected to have any use in the near future.

To simplify the current codebase and to remove a big chunk of unused code, any reference to javascript specific commands should be removed.