[RISCV] Remove Group<m_riscv_Features_Group> from mcmodel_EQ_medlow/mcmodel_EQ_medany. NFCI

As far as I can tell m_riscv_Features_Group is for options that
can be passed to cc1's -target-feature. These mcmodel aliases aren't
that.

I think they get treated as aliases and disappear before we get
to processing m_riscv_Features_Group so it didn't cause any problems.

I've left them in m_Group to match mcmodel_EQ.