Fix redundant access-level modifiers.

[dingobye]

This patch aims to make corelibs-xctest adapted to this PR, which produces warnings for redundant access-level modifiers.

For the code pattern shown in the example below, there are two feasible ways to fix the redundancy issue:

public extension {            // (1) remove `public` modifier for extension;
   public func foo() {}       // or (2) remove `public` modifier for member.
}

This patch goes with (2), keeping consistency with the rest of the project.

[stmontgomery]

@swift-ci Please test

[dingobye]

Thank you for firing the test, Stuart! Looks like everything goes well. It would be greatly appreciated if you could review and help to land this PR.

[stmontgomery]

This looks good to me, and matches the existing conventions within the project

[dingobye]

Thank you, Stuart!