Speculative password import prompt crash fix (#1072)

Please review the release process for BrowserServicesKit
[here](https://app.asana.com/0/1200194497630846/1200837094583426).

**Required**:

Task/Issue URL:
https://app.asana.com/0/1203822806345703/1208648341254407/f
iOS PR: duckduckgo/iOS#3569
macOS PR: duckduckgo/macos-browser#3542
What kind of version bump will this require?: Patch

**Description**:

There's a crash in moderate numbers which is occurring during the
password import prompt flow. This is speculative fix for that.

The reason seemed to be that the `replyHandler` was being called too
many times. However:
- The docs for
`userContentController:didReceiveScriptMessage:replyHandler:` say " The
replyHandler can be called at most once. If the replyHandler is
deallocated before it is called, the Promise will be rejected with a
JavaScript Error object
 with an appropriate message indicating the handler was never called."
- In the case of `startCredentialsImportFlow` where the crash is
occurring, the JS is not awaiting. So there is no need to call the
replyHandler.

**Steps to test this PR**:
Just smoke tests for the feature:
https://app.asana.com/0/1142021229838617/1208543152554655/f

**OS Testing**:

* [ ] iOS 14
* [ ] iOS 15
* [ ] iOS 16
* [ ] macOS 10.15
* [ ] macOS 11
* [ ] macOS 12

---
###### Internal references:
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)

Sources/BrowserServicesKit/Autofill/AutofillUserScript+SecureVault.swift

@@ -748,7 +748,6 @@ extension AutofillUserScript {
                 if !credentials.isEmpty {
                     self?.passwordImportDelegate?.autofillUserScriptDidFinishImportWithImportedCredentialForCurrentDomain()
                 }
-                replyHandler(nil)
             })
         }
     }