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

@@ -6,22 +6,22 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
-function dut {
-  (( $# == 0 )) && set -- *
+# function dut {
 
-  if grep -q -i 'GNU' < <(du --version 2>&1); then
-    du -khsc "$@" | sort -h -r
-  else
-    local line size name
-    local -a record
+(( $# == 0 )) && set -- *
 
-    while IFS=$'\n' read line; do
-      record=(${(z)line})
-      size="$(($record[1] / 1024.0))"
-      name="$record[2,-1]"
-      printf "%9.1LfM    %s\n" "$size" "$name"
-    done < <(du -kcs "$@") | sort -n -r
-  fi
-}
+if [[ ${(@M)${(f)"$(du --version 2>&1)"}:#*GNU *} ]]; then
+  du -khsc "$@" | sort -h -r
+else
+  local line size name
+  local -a record
 
-dut "$@"
+  while IFS=$'\n' read line; do
+    record=(${(z)line})
+    size="$(($record[1] / 1024.0))"
+    name="$record[2,-1]"
+    printf "%9.1LfM    %s\n" "$size" "$name"
+  done < <(du -kcs "$@") | sort -n -r
+fi
+
+# }