Remove EXECUTORCH_SEPARATE_FLATCC_HOST_PROJECT (#10860)

### Summary
I forgot to do this in https://github.com/pytorch/executorch/pull/10855.
But since we build flatcc for the host now, we don't need this variable.

### Test plan

CI

```
$ rg EXECUTORCH_SEPARATE_FLATCC_HOST_PROJECT --hidden -g '!.git/'
```

cc @larryliu0820