python: don't wipe RDEPENDS when parsing manifest

python: don't wipe RDEPENDS when parsing manifest

We want to allow the main recipe to extend RDEPENDS directly, so don't wipe
RDEPENDS when reading the manifest.

This fixes the missing python-misc dependency from python-modules.

As the wiping was having the good side-effect of removing the PN-dev dependency
on PN (which doesn't exist), clear RDEPENDS_${PN}-dev.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>