tty: powerpc: remove hvc_iseries

The PowerPC legacy iSeries platform is being removed, so this code is no
longer needed.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>