Remove description of the dependencies on Mac OS X

According to [the documentation of WebP](https://developers.google.com/speed/webp/docs/precompiled#getting_cwebp_dwebp_and_the_webp_libraries):

> Conversely, the libpng, libjpeg and libtiff libraries have been statically linked in the Mac OS X binaries. They do not require additional installation.

So we can use `cwebp` without any dependencies on Mac OS X.