Remove checks for LLVM < 4.0

While we still have to support LLVM 4.0 for Emscripten, we can
drop checks for LLVM >= 4.0 and < 4.0.

src/rustllvm/rustllvm.h

@@ -57,12 +57,8 @@
 
 #include "llvm/IR/LegacyPassManager.h"
 
-#if LLVM_VERSION_GE(4, 0)
 #include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/Bitcode/BitcodeWriter.h"
-#else
-#include "llvm/Bitcode/ReaderWriter.h"
-#endif
 
 #include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DebugInfo.h"