Remove unused "using" (NFC)

Identified by misc-unused-using-decls.

Author: kazutakahirata

lldb/source/Core/IOHandlerCursesGUI.cpp

@@ -78,8 +78,6 @@
 
 using namespace lldb;
 using namespace lldb_private;
-using llvm::None;
-using llvm::Optional;
 using llvm::StringRef;
 
 // we may want curses to be disabled for some builds for instance, windows

llvm/lib/InterfaceStub/ELFObjHandler.cpp

@@ -19,7 +19,6 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Process.h"
 
-using llvm::MemoryBufferRef;
 using llvm::object::ELFObjectFile;
 
 using namespace llvm;