_artifactcache/artifactcache.py: Silence lint issues

The pylint tool is unable to understand that the abstract methods
in the artifact cache will only be called when there's concrete
implementations behind them.  Silence the lint errors for these
specific calls.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

buildstream/_artifactcache/artifactcache.py

@@ -252,7 +252,7 @@ class ArtifactCache():
     #    (int): The size of the cache after having cleaned up
     #
     def clean(self):
-        artifacts = self.list_artifacts()
+        artifacts = self.list_artifacts()  # pylint: disable=assignment-from-no-return
 
         # Build a set of the cache keys which are required
         # based on the required elements at cleanup time
@@ -294,7 +294,7 @@ class ArtifactCache():
             if key not in required_artifacts:
 
                 # Remove the actual artifact, if it's not required.
-                size = self.remove(to_remove)
+                size = self.remove(to_remove)  # pylint: disable=assignment-from-no-return
 
                 # Remove the size from the removed size
                 self.set_cache_size(self._cache_size - size)
@@ -311,7 +311,7 @@ class ArtifactCache():
     #    (int): The size of the artifact cache.
     #
     def compute_cache_size(self):
-        self._cache_size = self.calculate_cache_size()
+        self._cache_size = self.calculate_cache_size()  # pylint: disable=assignment-from-no-return
 
         return self._cache_size