powerpc/powernv: Remove never used pnv_power9_force_smt4

This removes never used symbol - pnv_power9_force_smt4.

Note that we might still want to add stubs for:
	void pnv_power9_force_smt4_catch(void);
	void pnv_power9_force_smt4_release(void);

Fixes: 7672691a "powerpc/powernv: Provide a way to force a core into SMT4 mode"
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>