Delete memstick images for PowerPC.

These images only ever worked on Apple Powermacs, which are now a very
old platform, and did so only for a very loose definition of "worked"
(they booted on a small subset of supported machines). Moreover, all
the machines they *did* boot on also would boot from a memstick made
by dd'ing an CD image to a flash drive. Since a flash drive prepared
in this way would also boot all the newer systems we support, the
memstick images were strictly less functional than the CD images, even
for booting from memory sticks.
Reviewed by: jhibbits
MFC after: 1 week

(cherry picked from commit 1af48800c20fadb07acca2ab930bb3bccd363edb)