Remove obsolete macOS build instructions

openssl is already installed through the Brewfile, and libffi needs no
environment variable anymore now that glib was fixed.

Homebrew/homebrew-core#44446

README.md

@@ -66,20 +66,6 @@ pip install virtualenv
 ``` sh
 sudo port install python27 py27-virtualenv cmake yasm llvm
 ```
-##### On macOS >= 10.11 (El Capitan), you also have to install OpenSSL
-
-``` sh
-brew install openssl
-
-export OPENSSL_INCLUDE_DIR="$(brew --prefix openssl)/include"
-export OPENSSL_LIB_DIR="$(brew --prefix openssl)/lib"
-
-./mach build ...
-```
-
-##### Note: Please make sure the required `PKG_CONFIG_PATH` environment variable is set the same as the one provided from `brew info libffi` ([Check this comment for more details](https://github.com/servo/servo/issues/23015#issuecomment-475050175)).
-
-If you've already partially compiled servo but forgot to do this step, run `./mach clean`, set the shell variables, and recompile.
 
 #### On Debian-based Linuxes