RISC-V: Update the assembler insn testcase.

Since the 0x57 is preserved for the vadd.vv instruction in the integration
branch, remove it to make sure the testcase can work.

gas/
	* testsuite/gas/riscv/insn.d: Remove 0x57 since it is preserved
	for vadd.vv instruction.
	* testsuite/gas/riscv/insn.s: Likewise.