package_*.bbclass: Simplify addtask

The package_write task was previously removed. Remove a remaining superfluous
reference to it.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
image.py: check file exists before deleting

When RM_OLD_IMAGE = "1", we delete old images but we didn't check they
actually exist...

[YOCTO #6029]

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>