[AArch64][SME] Remove combination of private-ZA and preserves_za. (#78563)

The new Clang attributes no longer support the combination of having a
private-ZA function that preserves ZA. The use of __arm_preserves("za")
means that ZA is shared and preserved.

There wasn't that much benefit to the special handling of this, because
in practice it only meant that we'd avoid restoring the lazy-save
afterwards, but it still needed setting up a lazy-save (with the
possibility of using a 0-sized buffer).

Perhaps a new attribute will be added in the future to support this
case, at which point we can revert back some of the changes removed in
this patch. But for now removing this code simplifies things.