[memory refactor][1/n] Rename Memory -> VectorIO, MemoryBanks -> VectorDBs (#828)

See https://github.com/meta-llama/llama-stack/issues/827 for the broader
design.

This is the first part:

- delete other kinds of memory banks (keyvalue, keyword, graph) for now;
we will introduce a keyvalue store API as part of this design but not
use it in the RAG tool yet.
- renaming of the APIs

llama_stack/distribution/distribution.py

@@ -32,8 +32,8 @@ def builtin_automatically_routed_apis() -> List[AutoRoutedApiInfo]:
             router_api=Api.safety,
         ),
         AutoRoutedApiInfo(
-            routing_table_api=Api.memory_banks,
-            router_api=Api.memory,
+            routing_table_api=Api.vector_dbs,
+            router_api=Api.vector_io,
         ),
         AutoRoutedApiInfo(
             routing_table_api=Api.datasets,