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

@@ -6,12 +6,13 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
-function dut {
-  (( $# == 0 )) && set -- *
+# function dut {
 
-  if grep -q -i 'GNU' < <(du --version 2>&1); then
+(( $# == 0 )) && set -- *
+
+if [[ ${(@M)${(f)"$(du --version 2>&1)"}:#*GNU *} ]]; then
   du -khsc "$@" | sort -h -r
-  else
+else
   local line size name
   local -a record
 
@@ -21,7 +22,6 @@ function dut {
     name="$record[2,-1]"
     printf "%9.1LfM    %s\n" "$size" "$name"
   done < <(du -kcs "$@") | sort -n -r
-  fi
-}
+fi
 
-dut "$@"
+# }