utility: Use Zsh native mechanism to detect GNU 'du'

Skip additional call to `grep` and use Zsh native mechanism to detect
GNU version of `du`.

Further, Remove redundant `function` clause as per Prezto convention.

modules/utility/functions/dut

@@ -6,10 +6,11 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
-function dut {
+# function dut {
+
 (( $# == 0 )) && set -- *
 
-  if grep -q -i 'GNU' < <(du --version 2>&1); then
+if [[ ${(@M)${(f)"$(du --version 2>&1)"}:#*GNU *} ]]; then
   du -khsc "$@" | sort -h -r
 else
   local line size name
@@ -22,6 +23,5 @@ function dut {
     printf "%9.1LfM    %s\n" "$size" "$name"
   done < <(du -kcs "$@") | sort -n -r
 fi
-}
 
-dut "$@"
+# }