Remove UIWebView references per ITMS-90809

[xaphod]

[REQUIRED] Step 2: Describe your environment

• Xcode version: 10.3
• Firebase SDK version: 6.7.0
• Firebase Component: DynamicLinks
• Component version: 4.0.3

[REQUIRED] Step 3:

This component references (allocs) UIWebView, which per ITMS-90809 is deprecated and throws warnings on app submission.

Specifically, see (only) FIRDLJavaScriptExecutor.m.

I am not sure if anyone is building apps on iOS 7 anymore when iOS 13 is right around the corner, but I am sure that Apple is going to soon start flat-out rejecting apps that instantiate UIWebViews (or make it a compile error in iOS 13 final, no clue).

Please remove all references to UIWebView, thanks!

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[paulb777]

Duplicate of #3722. The fix is going out in this weeks Firebase 6.8.0 (M55) release

[xaphod]

Thanks and sorry: I forgot to search closed issues (didn't occur to me). Lesson learned.