.zshrc: Remove vimdiff alias on Mac OS X

This was only necessary when I was using vim from pkgsrc. The one
from Homebrew provides a `vimdiff` symlink.

.zshrc

@@ -51,8 +51,6 @@ if [[ "$OSTYPE" =~ ^darwin.*$ ]]; then
     alias dircolors='gdircolors'
     alias sort='gsort'
     alias grep='ggrep'
-    # pkgsrc doesn't provide vimdiff wrapper, but it's just `vim -d` anyways
-    alias vimdiff='vim -d'
 
     # If BasicTeX installation exists, add it to PATH
     # See: https://tug.org/mactex/morepackages.html