Revert "Attempted workaround for macOS SCons build failure"

This reverts commit 643210b.

The workaround didn't help. SCons still doesn't find libffi. See build
log at https://travis-ci.org/wesnoth/wesnoth/jobs/505677825

utils/travis/steps/install.sh

@@ -23,7 +23,7 @@ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
         brew install ccache
         travis_wait ./projectfiles/Xcode/Fix_Xcode_Dependencies
     else
-        brew install scons libffi cairo pango moreutils sdl2_image sdl2_ttf sdl2_mixer glew ccache
+        brew install scons cairo pango moreutils sdl2_image sdl2_ttf sdl2_mixer glew ccache
     fi
 else
     if [ "$NLS" != "true" ]; then