x86/mem_encrypt: Remove stale mem_encrypt_init() declaration

The memory encryption initialization logic was moved from init/main.c
into arch_cpu_finalize_init() in commit 439e1757 ("init, x86: Move
mem_encrypt_init() into arch_cpu_finalize_init()"), but a stale
declaration for the init function was left in <linux/init.h>.

And didn't cause any problems if you had X86_MEM_ENCRYPT enabled, which
apparently everybody involved did have.  See also commit 0a9567ac
("x86/mem_encrypt: Unbreak the AMD_MEM_ENCRYPT=n build") in this whole
sad saga of conflicting declarations for different situations.

Reported-by: Matthew Wilcox <willy@infradead.org>
Fixes: 439e1757 init, x86: Move mem_encrypt_init() into arch_cpu_finalize_init()
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>