qapi: Delete all the "foo: dropped in n.n" notes

qapi: Delete all the "foo: dropped in n.n" notes

A handful of QAPI doc comments include lines like
"ppcemb: dropped in 3.1". The doc comment parser will just
put these into whatever the preceding section was; sometimes
that's "Notes", and sometimes it's some random other section,
as with "NetClientDriver" where the "'dump': dropped in 2.12"
line ends up in the "Since:" section.

This tends to render wrongly, more so in the upcoming rST
generator, but sometimes even in the Texinfo, as in the case
of QKeyCode:
   ac_bookmarks
       since 2.10 altgr, altgr_r: dropped in 2.10

Since commit 3264ffced3 (v4.2.0), we have a better place to tell
users about deprecated and deleted functionality --
qemu-deprecated.texi.  These "dropped in" remarks all predate it, and
other feature drops of that vintage are not documented anywhere, so
moving these to qemu-deprecated.texi makes little sense.  Drop them
instead.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200213175647.17628-19-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>