Drop PRIORITY variable

As discussed on the mailing list, this variable isn't useful and if wanted
would be better implemented by distros using pn-X overrides.

This patch executes:

find . -regex ".*\.\(bb\|inc\)$" | xargs sed -i '/^PRIORITY = ".*"$/d'

against the tree removing the references. Thanks to Phil Blundell for
the command.

(Prompted by oe-core rev: d122343362669c683acc4af295971a62cbc823fc)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>