target/sparc: Remove old TODO file

This file hasn't seen a real (non-trivial) update since 2008 anymore,
so we can assume that it is pretty much out of date and nobody cares
for it anymore. Let's simply remove it.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Artyom Tarasenko <atar4qemu@gmail.com>
Message-Id: <20190930171044.25312-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>