KVM: arm64: Remove unused __extended_idmap_trampoline() prototype

__extended_idmap_trampoline() was removed a long time ago by
3421e9d8

 ("arm64: KVM: Simplify HYP init/teardown") so remove the
unused function prototype.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201118194402.2892-4-will@kernel.org