docs: Remove OS X pyspotify installation warning

I've just tested the following combinations:

- OS X 10.9.3, Xcode 5.1.1, Apple Python 2.7.5: OK without workaround
- OS X 10.9.3, Xcode 5.1.1, Homebrew Python 2.7.7_2: OK without workaround

Thus, I'm removing the warning from the docs.