powerpc: Remove obsoleted declarations for use_cop and drop_cop

The use_cop() and drop_cop() have been removed since
commit 6ff4d3e9 ("powerpc: Remove old unused icswx based
coprocessor support"), now they are useless, so remove them.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240822130609.786431-5-cuigaosheng1@huawei.com

arch/powerpc/include/asm/mmu_context.h

@@ -116,9 +116,6 @@ static inline bool need_extra_context(struct mm_struct *mm, unsigned long ea)
 }
 #endif
 
-extern int use_cop(unsigned long acop, struct mm_struct *mm);
-extern void drop_cop(unsigned long acop, struct mm_struct *mm);
-
 #ifdef CONFIG_PPC_BOOK3S_64
 static inline void inc_mm_active_cpus(struct mm_struct *mm)
 {