Remove login import de-duplication feature flag (#1000)

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

**Required**:

Task/Issue URL:
https://app.asana.com/0/414235014887631/1207759292115042/f
iOS PR: duckduckgo/iOS#3369
macOS PR: duckduckgo/macos-browser#3313
What kind of version bump will this require?: Major

**Description**:

For [✓ macOS: Deduplicate passwords on
import](https://app.asana.com/0/0/1207597760316574), a feature flag was
added to mitigate risk of negative user reaction / unforeseen issues.
This should be removed once we're confident in the implementation.

The feature flag is named deduplicateLoginsOnImport

**Steps to test this PR**:
1. See macOS PR

**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/PrivacyConfig/Features/PrivacyFeature.swift

@@ -80,7 +80,6 @@ public enum AutofillSubfeature: String, PrivacySubfeature {
     case accessCredentialManagement
     case autofillPasswordGeneration
     case onByDefault
-    case deduplicateLoginsOnImport
     case onForExistingUsers
     case unknownUsernameCategorization
 }