powerpc/mm: Drop unused externs for hpte_init_beat[_v3]()

We removed the BEAT support in 2015 in commit bf4981a0 ("powerpc:
Remove the celleb support"). These externs are unused since then.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/mmu-hash.h

@@ -398,8 +398,6 @@ static inline void hpte_init_pseries(void) { }
 #endif
 
 extern void hpte_init_native(void);
-extern void hpte_init_beat(void);
-extern void hpte_init_beat_v3(void);
 
 extern void slb_initialize(void);
 extern void slb_flush_and_rebolt(void);