Get rid of G_VFY_TOKEN_HDR_IGNORE_SEQ_SIZE

This flag was introduced in the mskrb-integ merge but is not actually
used after r21742--while kg_unseal_iov_token sets it in vfyflags for
DCE-style contexts, it doesn't actually pass vfyflags to
g_verify_token_header or otherwise use it.  Moreover, the flag is not
necessary there; we correctly set input_length to the header length
(without data, padding, or trailer) for v1 tokens in a DCE-style
context.