KVM: remove unused guest_enter

After commit 61bd0f66 ("KVM: PPC: Book3S HV: Fix guest time accounting
with VIRT_CPU_ACCOUNTING_GEN"), no one use this function anymore, So better
to remove it.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>