Don't remove build directory when building dependencies (needed when CLEANDATE is set)

Ghost User
2018-11-13T16:06:30+01:00

When building dependencies nestedly (as required when `CLEANDATE` is set), the `build` directory while it is still needed in later steps.

Thus the following build would fail:

```
export CLEANDATE=2018-10-23
./generic/setup.sh clean-lib-sapl linux x64
./generic/fetch.sh clean-lib-sapl linux x64
./generic/build.sh clean-lib-sapl linux x64
```

Because the `build/clean` directory would not contain `etc` any more while it is used in `build.sh`.