[clang-fuzzer] Remove 'setUseOrcMCJITReplacement(false)' call.

The default value for this option (UseMCJITReplacement) is already false, and
OrcMCJITReplacement is going to have deprecation warnings attached in LLVM 9.0.
Removing this call removes a spurious warning.

llvm-svn: 366141