utility: Remove redundant `function` clause in 'diff'

We don't use `function` clause in Prezto by convention.

modules/utility/functions/diff

@@ -5,13 +5,13 @@
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
 
-function diff {
+# function diff {
-  if zstyle -t ':prezto:module:utility:diff' color \
+
+if zstyle -t ':prezto:module:utility:diff' color \
       && (( $+commands[colordiff] )); then
   command colordiff "$@"
-  else
+else
   command diff "$@"
-  fi
+fi
-}
 
-diff "$@"
+# }