intersil: remove obsolete prism54 wireless driver

Commit 1d89cae1 ("MAINTAINERS: mark prism54 obsolete") indicated the
prism54 driver as obsolete in July 2010.

Now, after being exposed for ten years to refactoring, general tree-wide
changes and various janitor clean-up, it is really time to delete the
driver for good.

This was discovered as part of a checkpatch evaluation, investigating all
reports of checkpatch's WARNING:OBSOLETE check.

p54 replaces prism54 so users should be unaffected. There was a one off chipset
someone long ago reported that p54 didn't work with but the reporter never
followed up on that. Additionally, distributions have been blacklisting prism54
for years now.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210713054025.32006-1-lukas.bulwahn@gmail.com