sparc64: Kill ISA_FLOPPY_WORKS code.

This never was enabled, I could never get it working, and if anyone
wants to try and get it's very easy to reference this code in the
history.

It's the only thing referencing the silly ISA device layer in the
sparc64 tree.  OF device layer infrastructure is what should be used
for these things.

Signed-off-by: David S. Miller <davem@davemloft.net>