Remove the function get_dynamic_sa_offset_iv_field() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.
Signed-off-by: NRickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>