classes/license: drop erroneous sha256 parameter in LIC_FILES_CHKSUM

classes/license: drop erroneous sha256 parameter in LIC_FILES_CHKSUM

In OE-Core commit a48fea275b08ff3d3dfc9a928aeb04768db35873, a check on
the value of a "sha256" parameter was added, however there was no
mention of this in the commit message and no corresponding code to
actually verify the checksum as sha256 was added along with it either,
so there's no point in getting the value. Additionally it was assuming
that a sha256 value would be present without checking first, with the
result that if you leave out the md5 value in a recipe intentionally in
order to get it to tell you the correct value on the next build, you got
a traceback instead of the appropriate error containing the information.
Drop this entirely - if we want to implement this we need to do it
properly.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>