[chore] Delete deprecated avatars (elastic#205448)

## Summary

As titled.  Delete deprecated avatars, replace stray usage.

<img width="1021" alt="Screenshot 2025-01-02 at 4 38 33 PM"
src="https://github.com/user-attachments/assets/01e8045c-5209-4926-a5da-f04aa65ac76e"
/>

x-pack/platform/plugins/shared/observability_solution/observability_ai_assistant/public/index.ts

@@ -32,7 +32,6 @@ export type {
 };
 
 export { aiAssistantCapabilities } from '../common/capabilities';
-export { AssistantAvatar } from './components/assistant_avatar';
 export { ConnectorSelectorBase } from './components/connector_selector/connector_selector_base';
 export { useAbortableAsync, type AbortableAsyncState } from './hooks/use_abortable_async';
 export { useGenAIConnectorsWithoutContext } from './hooks/use_genai_connectors';

x-pack/solutions/observability/plugins/observability_ai_assistant_app/public/components/rca/rca_callout/index.tsx

@@ -19,8 +19,8 @@ import {
 } from '@elastic/eui';
 import React from 'react';
 import { i18n } from '@kbn/i18n';
-import { AssistantAvatar } from '@kbn/observability-ai-assistant-plugin/public';
 import { css } from '@emotion/css';
+import { AssistantIcon } from '@kbn/ai-assistant-icon';
 
 export function RootCauseAnalysisCallout({
   onClick,
@@ -46,7 +46,7 @@ export function RootCauseAnalysisCallout({
             `}
           >
             <EuiFlexGroup direction="row" alignItems="center" justifyContent="center">
-              <AssistantAvatar />
+              <AssistantIcon size="l" />
               <EuiFlexItem grow={false}>
                 <EuiTitle size="xs">
                   <h2>