Remove warning for 'unlikely' macro redefinition

#### Summary:

Remove warning:
```
stan/src/stan/memory/stack_alloc.hpp:17:9: warning: 'unlikely' macro redefined
#define unlikely(x) (__builtin_expect((x),0))
        ^
stan/src/stan/agrad/fwd/fvar.hpp:6:9: note: previous definition is here
#define unlikely(x) (x)
        ^
```

#### Intended Effect:

Removes warning, but should not change behavior.

#### How to Verify:

Check the compile logs to see if there are warnings.

#### Side Effects:

None. Unless something was actually using the redefined `unlikely` macro in fvar.hpp.

#### Documentation:

Not user facing.

#### Reviewer Suggestions: 

Anyone.