struct flow_entry does not need field in_use

The field `in_use` of `struct flow_entry` can be replaced with the following inline function:
```c
static inline bool
flow_entry_in_use(const struct flow_entry *fe)
{
return fe->grantor_fib != NULL;
}
```
The field `grantor_fib` must be set for GK blocks to know how to forward packets to Grantor servers or the actual destination, so it is always set for entries in use.

Dropping field `in_use` is just one more step to streamline `struct flow_entry`.