[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/agents/agents.py

@@ -33,7 +33,6 @@ from llama_stack.apis.inference import (
     ToolResponseMessage,
     UserMessage,
 )
-from llama_stack.apis.memory import MemoryBank
 from llama_stack.apis.safety import SafetyViolation
 from llama_stack.apis.tools import ToolDef
 from llama_stack.providers.utils.telemetry.trace_protocol import trace_protocol
@@ -133,8 +132,6 @@ class Session(BaseModel):
     turns: List[Turn]
     started_at: datetime
 
-    memory_bank: Optional[MemoryBank] = None
-
 
 class AgentToolGroupWithArgs(BaseModel):
     name: str