[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/apis/resource.py

@@ -14,7 +14,7 @@ from pydantic import BaseModel, Field
 class ResourceType(Enum):
     model = "model"
     shield = "shield"
-    memory_bank = "memory_bank"
+    vector_db = "vector_db"
     dataset = "dataset"
     scoring_function = "scoring_function"
     eval_task = "eval_task"