rpm: remove tmp folder created during install

rpm: remove tmp folder created during install

nativesdk-rpm build is also affected by the stray /var/tmp
created during the rpm install. Remove it to fix QA Issue:
nativesdk-rpm installs files in /usr/local/oe-sdk-hardcoded-buildpath/sysroots/x86_64-pokysdk-linux/var/volatile, but it is expected to be empty [empty-dirs]

Signed-off-by: Samuli Piippo <samuli.piippo@qt.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>