Always (re)set LS_COLORS

Prior to this change, if I changed the dircolors configuration and
restarted the shell, it wouldn't reflect in the new shell because of the
check. I'd have to create a new terminal window to get the new colors.

This change eliminates the check for a set LS_COLORS variable and always
calls dircolors.

zsh/lscolors.zsh

@@ -1,10 +1,4 @@
 # Add ls coloring
-# Don't bother setting LS_COLORS if it's alredy set
-if [[ -n "$LS_COLORS" ]]
-then
-    return
-fi
-
 if [[ "`uname`" == *"Darwin"* ]]
 then
     # OS X uses BSD ls which is relatively restricted compared to