stdlib: Remove some redundant generic requirements

[slavapestov]

The GSB didn't emit warnings for these for some reason. There's no harm in leaving them in, but you'll get warnings now.

Proof that they're really redundant:

• _ArrayBufferProtocol declares where Indices == Range<Int> already so there's no point restating it in the extension.
• Result requires that Failure conform to Error which inherits from Sendable, so again it's redundant to restate in the extension.

[slavapestov]

@swift-ci Please smoke test