doc: Get rid of 'static' property container in jsduck index

Us grouping the inheritable static properties that way is an
implementation detail that is polluting the index and makes
it harder to refer to individual identifiers.

It also causes problems under JSDuck 5 because that version is
more strict about defining properties (Foo.static.bar) of which
the parent is not defined in the index (Foo.static), we'd have
to add a sea of `@static @property {Object} this.static` all
over the place. Might as well hide this implementation detail
and just consider them static properties (just like we already
do for "private" properties).

Change-Id: Ibf2ebf7752aabc2b75b6ac6fa00e2284a181a600