[Lanai] Remove redundant declaration getTheLanaiTarget (NFC)

Note that getTheLanaiTarget is declared in
TargetInfo/LanaiTargetInfo.h, which LanaiDisassembler.cpp includes.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/Target/Lanai/Disassembler/LanaiDisassembler.cpp

@@ -26,10 +26,6 @@ using namespace llvm;
 
 typedef MCDisassembler::DecodeStatus DecodeStatus;
 
-namespace llvm {
-Target &getTheLanaiTarget();
-}
-
 static MCDisassembler *createLanaiDisassembler(const Target & /*T*/,
                                                const MCSubtargetInfo &STI,
                                                MCContext &Ctx) {