package_rpm.bbclass: Remove empty build directory

package_rpm.bbclass: Remove empty build directory

Fixed:
$ bitbake acl
$ ls tmp/work/core2-64-poky-linux/acl/2.3.2/build/acl-2.3.2-build/

This empty directory is created by rpmbuild, just remove it like others above.

Note, master branch doesn't have this problem after upgrade to rpm 4.20.1, but
it's not easy to find which patch(es) fixed the problem, and the issues haven't
been fixed completely, there is still an empty SRPMS directory, so just remove
the build directory as others above.

If we don't fix this, there might be warnings when a recipe uses:

install -m 0644 ${B}/* ${DEPLOYDIR}/boot.bin-extracted/

install: omitting directory '${B}/xilinx-bootbin-1.0-build'

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>