[RISCV] Fix test/MC/Disassembler/RISCV/invalid-instruction.txt after rL347988

The test for [0x00 0x00] failed due to the introduction of c.unimp.

This particular test is unnecessary now that c.unimp was defined (and is 
tested in test/MC/RISCV/rv32c-valid.s).

llvm-svn: 348117