Removed unnecessary cmake osx deployment target value

[JonnyPtn]

• Has this change been discussed on the forum or in an issue before?
• Does the code follow the SFML Code Style Guide?
• Have you provided some example/test code for your changes?

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Description

Addresses #1436

Tasks

• Tested on macOS
• Tested on iOS

How to test this PR?

If you're on macOS, check this doesn't break your build

[ssrobins]

I took the code from osx_deploy_target branch in the JonnyPtn/SFML repo and built it in my build system, using CMake to generate Makefiles and Xcode project files.

I commented out this line in my build system, which I was using as a workaround to the reported difficulty I was having:
set(CMAKE_OSX_DEPLOYMENT_TARGET "10.9" CACHE STRING "Minimum OS X deployment version" FORCE)

SFML and all games that use it built fine in both configurations. gtest, which had issues with the 10.7 target setting, built fine too.

My environment:
MacBook pro 13-inch retina, early 2015
macOS 10.13.6
Xcode 9.4.1

[eXpl0it3r]

Merged in e3c6509