Drop the deprecated lm32 target

Target lm32 was deprecated in commit d8498005, v5.2.0.  See there
for rationale.

Some of its code lives on in device models derived from milkymist
ones: hw/char/digic-uart.c and hw/display/bcm2835_fb.c.

Cc: Michael Walle <michael@walle.cc>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210503084034.3804963-2-armbru@redhat.com>
Acked-by: Michael Walle <michael@walle.cc>
[Trivial conflicts resolved, reST markup fixed]