doc: Remove the obsolete README.mpc74xx file

Support for the PPC74xx processors has been removed in commit
d928664f ("powerpc: 74xx_7xx: remove 74xx_7xx cpu support")
more than 6 years ago already. So the corresponding README file
can now be removed, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>