x86/32: Remove unused IA32_STACK_TOP and two externs

Since

  32974ad4 ("[IA64] Remove COMPAT_IA32 support")

IA32_STACK_TOP and ia32_setup_arg_pages() are not used anymore.

And

  675a0813 ("x86: unify mmap_{32|64}.c")

left behind ia32_pick_mmap_layout() extern declaration.

Remove them all.

  [ bp: Massage commit message. ]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230625125411.25840-1-yuehaibing@huawei.com