package.bbclass: Fix sources contents

http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=c376f1f49cea182a2887945840ab97a20970a373
fixed a valid issue where the sources file was accumulating information
and subsequent task runs of do_packacge were not cleaning it. The
fix is wrong however since we're removing the file within a loop.
This fix removes the file outside the loop ensuring it is not truncated
and contains the correct information.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>