refactor(cw): remove unused keep-alive workaround. (#6694)

## Problem
This code is no longer used, and appears to have been moved to the LSP
side:
aws/aws-toolkit-vscode-staging#1214 (comment)

This fix also no longer works, and the work it was based off has
significantly changed their approach:
https://github.com/sourcegraph/cody/blob/62d73f78c432036d1f99bc9631ed534cc2ed846b/vscode/src/net/net.patch.ts

## Solution
- remove it. 


---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

packages/core/src/codewhisperer/activation.ts

@@ -83,7 +83,6 @@ import { CodeWhispererCommandBackend, CodeWhispererCommandDeclarations } from '.
 import { SecurityIssueHoverProvider } from './service/securityIssueHoverProvider'
 import { SecurityIssueCodeActionProvider } from './service/securityIssueCodeActionProvider'
 import { listCodeWhispererCommands } from './ui/statusBarMenu'
-import { updateUserProxyUrl } from './client/agent'
 import { Container } from './service/serviceContainer'
 import { debounceStartSecurityScan } from './commands/startSecurityScan'
 import { securityScanLanguageContext } from './util/securityScanLanguageContext'
@@ -201,10 +200,6 @@ export async function activate(context: ExtContext): Promise<void> {
                     })
             }
 
-            if (configurationChangeEvent.affectsConfiguration('http.proxy')) {
-                updateUserProxyUrl()
-            }
-
             if (configurationChangeEvent.affectsConfiguration('amazonQ.ignoredSecurityIssues')) {
                 const ignoredIssues = CodeWhispererSettings.instance.getIgnoredSecurityIssues()
                 toggleIssuesVisibility((issue) => !ignoredIssues.includes(issue.title))

packages/core/src/codewhisperer/client/codewhisperer.ts

@@ -20,11 +20,11 @@ import userApiConfig = require('./user-service-2.json')
 import { session } from '../util/codeWhispererSession'
 import { getLogger } from '../../shared/logger/logger'
 import { indent } from '../../shared/utilities/textUtilities'
-import { keepAliveHeader } from './agent'
 import { getClientId, getOptOutPreference, getOperatingSystem } from '../../shared/telemetry/util'
 import { extensionVersion, getServiceEnvVarConfig } from '../../shared/vscode/env'
 import { DevSettings } from '../../shared/settings'
 
+const keepAliveHeader = 'keep-alive-codewhisperer'
 export interface CodeWhispererConfig {
     readonly region: string
     readonly endpoint: string