UBUNTU: [Packaging] final-checks -- remove check for empty retpoline files

UBUNTU: [Packaging] final-checks -- remove check for empty retpoline files

With the new support for removing known safe retpoline sequences
from the ones which were detected it is now completely valid to
end up with an empty retpoline file in the abi. Remove the
check for empty retpoline files so this will not cause an error.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>