QDoc: Drop unused QML markup attribute support

QML syntax highlighting has no use for markup attributes. Its unused
attribute handling is also incorrect: it treats attribute values as
names.

Remove this unused, broken support to simplify QML syntax highlighting.
Other code markers still emit attributes, so generators retain their
support for them.

Pick-to: 6.12
Change-Id: I554cc19d11a7ab9de13ed89b569cb63fb9076e9d
Reviewed-by: Paul Wicking <paul.wicking@qt.io>