[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

Author: asb

llvm/test/MC/Disassembler/RISCV/invalid-instruction.txt

@@ -4,10 +4,6 @@
 # Test generated by a LLVM MC Disassembler Protocol Buffer Fuzzer
 # for the RISC-V assembly language.
 
-# This should not decode as c.addi4spn with 0 imm when compression is enabled.
-[0x00 0x00]
-# CHECK: warning: invalid instruction encoding
-
 # This should not decode as c.addi16sp with 0 imm when compression is enabled.
 [0x01 0x61]
 # CHECK: warning: invalid instruction encoding