x86/efistub: Drop redundant clearing of BSS

As it turns out, clearing the BSS was not the right fix for the issue
that was ultimately fixed by commit decd347c ("x86/efistub:
Reinstate soft limit for initrd loading"), and given that the Windows
EFI loader becomes very unhappy when entered with garbage in BSS, this
is one thing that x86 PC EFI implementations can be expected to get
right.

So drop it from the pure PE entrypoint. The handover protocol entrypoint
still needs this - it is used by the flaky distro bootloaders that
barely implement PE/COFF at all.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>