[iOS][Feature Flags] Remove unused onboarding add to dock feature flag (#2679)

Task/Issue URL: https://app.asana.com/1/137249556945/project/1211834678943996/task/1211866468574274?focus=true
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### Description

Removed the unused feature flag for onboarding add to dock from the
codebase. The onboarding add-to-dock feature was released to all users a
long time ago, so the feature flag is no longer needed.

### Testing Steps
1. The feature flag was not used anywhere in the codebase so ensure only
CI is green.

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### Impact and Risks
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### Quality Considerations
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---
###### Internal references:
[Definition of
Done](https://app.asana.com/0/1202500774821704/1207634633537039/f) |
[Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552) |
[Tech Design
Template](https://app.asana.com/0/59792373528535/184709971311943)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Removes the unused `onboardingAddToDock` feature flag and its
references from `iOS/Core/FeatureFlag.swift`.
> 
> - **iOS/Core**:
> - Remove `FeatureFlag.onboardingAddToDock` enum case in
`iOS/Core/FeatureFlag.swift`.
>   - Drop `onboardingAddToDock` from `supportsLocalOverriding`.
>   - Remove `onboardingAddToDock` handling from `source` mapping.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
c268a85. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: alessandroboron

iOS/Core/FeatureFlag.swift

@@ -96,9 +96,6 @@ public enum FeatureFlag: String {
     /// https://app.asana.com/1/137249556945/project/1211834678943996/task/1211866468468677
     case onboardingHighlights
 
-    /// https://app.asana.com/1/137249556945/project/1211834678943996/task/1211866468574274
-    case onboardingAddToDock
-
     /// https://app.asana.com/1/137249556945/project/1211834678943996/task/1211866711364768
     case autofillSurveys
 
@@ -394,7 +391,6 @@ extension FeatureFlag: FeatureFlagDescribing {
                .syncPromotionBookmarks,
                .syncPromotionPasswords,
                .onboardingHighlights,
-               .onboardingAddToDock,
                .autofillSurveys,
                .autocompleteTabs,
                .adAttributionReporting,
@@ -466,8 +462,6 @@ extension FeatureFlag: FeatureFlagDescribing {
             return .remoteReleasable(.subfeature(SyncPromotionSubfeature.passwords))
         case .onboardingHighlights:
             return .internalOnly()
-        case .onboardingAddToDock:
-            return .internalOnly()
         case .autofillSurveys:
             return .remoteReleasable(.feature(.autofillSurveys))
         case .autocompleteTabs: