powerpc/mm: Remove __this_fixmap_does_not_exist()

This function has not been used since commit 9494a1e842
("powerpc: use generic fixmap.h)

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/pgtable_32.c

@@ -390,8 +390,3 @@ void __set_fixmap (enum fixed_addresses idx, phys_addr_t phys, pgprot_t flags)
 	map_page(address, phys, pgprot_val(flags));
 	fixmaps++;
 }
-
-void __this_fixmap_does_not_exist(void)
-{
-	WARN_ON(1);
-}