Revert "base-files: fwtool: Fix wrong checksum on combined-image with metadata"

Revert "base-files: fwtool: Fix wrong checksum on combined-image with metadata"

This reverts commit 41770add03ad77a0ce41ed424ad050238f7d9272.

The fwtool_check_image() procedure is used by `sysupgrade --test` which must
not alter the image under test in any way.

Currently, when the LuCI ui or any other sysupgrade wrapper first invokes
sysupgrade --test to verify the compatibility of the image and then calculates
the sha256sum over it, the resulting checksum will differ from the original
image since the test invocation will implicitely strip the metadata trailer.

To properly fix the underlying issue, the combined image checksumming code
must be modified to skip the metadata trailer.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>