[BPF] Remove unused declaration probeJmpExt

The declaration was added without a corresponding function definition
by:

  commit dc1dbf6
  Author: Yonghong Song <[email protected]>
  Date:   Wed Aug 23 04:25:57 2017 +0000

Author: kazutakahirata

llvm/lib/Target/BPF/BPFSubtarget.h

@@ -38,7 +38,6 @@ class BPFSubtarget : public BPFGenSubtargetInfo {
 private:
   void initializeEnvironment();
   void initSubtargetFeatures(StringRef CPU, StringRef FS);
-  bool probeJmpExt();
 
 protected:
   // unused