target/riscv: rvv-1.0: slide instructions

* Remove clear function from helper functions as the tail elements
  are unchanged in RVV 1.0.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211210075704.23951-51-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>