lib/oe/package_manager: ensure repodata is wiped

When building package indexes for RPM feeds, delete repodata first to
be absolutely sure that the indexes are up to date.

We've seen some mysterious failures where the repodata doesn't match
the repository, so hopefully this will stop that happening.

[ YOCTO #14190 ]

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