[BOLT][NFC] Address warning about ProgramPoint implicit copy constructor

Summary:
Explicit assignment operator can be replaced with an implicit one.
Remove it to allow an implicit copy constructor:
```
bolt/src/Passes/DataflowAnalysis.h:74:8: warning: definition of
implicit copy constructor for 'ProgramPoint' is deprecated because it
has a user-declared copy assignment operator [-Wdeprecated-copy]
  void operator=(const ProgramPoint &PP) {
       ^
bolt/src/Passes/DataflowAnalysis.h:62:14: note: in implicit copy
constructor for 'llvm::bolt::ProgramPoint' first required here
      return ProgramPoint(&*Last);
```

(cherry picked from FBD28335138)