Remove macOS @available version restrictions.

[spevans]

Im not sure if this is valid, but restricting some methods to certain versions of macOS doesn't seem necessary here.

At the same time, some of there if #available that rely on Foundation don't seem to be needed given that they are contained in this version of Foundation.

Tested on linux and macOS 10.12

@parkera @phausler Is this ok or am I on the wrong track here?

[phausler]

This will likely break our capability of copy/paste to and from the overlay

[parkera]

I agree with @phausler on trying to keep the source code in sync; that's the best option we have while we need to have these things in separate repositories.

I assume that generated interfaces would show these APIs as available since Linux and other platforms fall under the *.

[alblue]

I concur - although these are no-ops on Linux, they will introduce problems in future sync updates, so leaving them as is will be more appropriate.

[spevans]

I had forgotten about the code-sync issue and the only real value of this PR was it slightly simplified some of the tests in the Darwin compatibiltity testing (#1286) so its not really that important.