phosphor: Remove 'pflash -i' service

Once upon a time this service was added to initalise the host flash
before starting the host firmware, otherwise some systems would not
boot.

 > commit 14f965d8
 > Author: Andrew Geissler <andrewg@us.ibm.com>
 > Date:   Thu May 11 09:54:04 2017 -0500
 >
 >     PNOR init workaround for P8 based systems
 >
 >     The PNOR chip requires an init to be done to it prior
 >     to starting the host.  This is a temporary fix to this
 >     issue until we get the mboxd function ported to P8.

Ever since e7b0fd56 ("Switch userspace to use MTD access by
default", October 2017) running pflash hs not done this, as it
access the flash through the kernel mtd interface.

A machine could chose use mboxd and the mbox protocol on P8 but it is
not required to fix this issue.

Change-Id: I5c2400063bf776018ef60f2642fb2629c96af0a7
Resolves: openbmc/openbmc#1598
Signed-off-by: Joel Stanley <joel@jms.id.au>