We now have a construct based encoder/decoder for the record content
of EF.MSISDN. This means that we do not need the functions enc_msisdn
and dec_msisdn in the non-legacy code anymore. We can now move both
to legacy/utils.py.
Related: OS#5714
Change-Id: I19ec8ba14551ec282fc0cc12ae2f6d528bdfc527