KVM: x86: remove unused declaration of kvm_write_tsc()

kvm_write_tsc() was renamed and made static since commit 0c899c25
("KVM: x86: do not attempt TSC synchronization on guest writes"). Remove
its unused declaration.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Message-Id: <20210326070334.12310-1-dongli.zhang@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>