Clean #ifdef around default RunOn declaration (#4580)

This PR replaces
```
#if defined(AMREX_USE_GPU)
    template <RunOn run_on>
#else
    template <RunOn run_on=RunOn::Host>
#endif
```
with
```
    template <RunOn run_on AMREX_DEFAULT_RUNON>
```

Author: AlexanderSinn