linux/local.conf.sample: Remove mips and ppc based machines

These machines are not as of now in the scope of the project. We remove
them from the local.conf samples to not leave room for confusion. This
aligns the qemu set of supported targets with `conf-notes.txt` and
general documentation too.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

flavours/linux/local.conf.sample

@@ -18,9 +18,6 @@
 #
 #MACHINE ?= "qemuarm"
 #MACHINE ?= "qemuarm64"
-#MACHINE ?= "qemumips"
-#MACHINE ?= "qemumips64"
-#MACHINE ?= "qemuppc"
 #MACHINE ?= "qemux86"
 #MACHINE ?= "qemux86-64"
 #MACHINE ?= "stm32mp1-av96"