lib.Distro: Fix mypy.truthy-bool fallout

The _select() method in Distro.py has an assert on a PackageFilter
parameter that doesn't define __bool__ or __len__. Since the type
signature already guarantees it is not None, the assertion is
redundant and is now removed.

Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev 0.81.1
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/Distro.py

@@ -364,7 +364,6 @@ class Distro(abc.ABC):
 
     async def _select(self, names: Iterable[str],
                       filter: PackageFilter) -> Iterable[Package]:
-        assert filter, 'No filter in _select()'
         return [p for p in await self._select_by_name(names) if filter.match(p)]
 
     async def select(