riscv: kvm: vcpu_timer: fix unused variable warnings

In two places, csr is set but never used:

arch/riscv/kvm/vcpu_timer.c:302:23: warning: variable 'csr' set but not used [-Wunused-but-set-variable]
        struct kvm_vcpu_csr *csr;
                             ^
arch/riscv/kvm/vcpu_timer.c:327:23: warning: variable 'csr' set but not used [-Wunused-but-set-variable]
        struct kvm_vcpu_csr *csr;
                             ^

Remove the variable.

Fixes: 8f5cb44b

 ("RISC-V: KVM: Support sstc extension")
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Anup Patel <anup@brainfault.org>