Remove fish completion script

fish ships with a much more capable rbenv completion script since fish 2.0.

Fixes #1212

completions/rbenv.zsh

@@ -1,18 +0,0 @@
-if [[ ! -o interactive ]]; then
-    return
-fi
-
-compctl -K _rbenv rbenv
-
-_rbenv() {
-  local words completions
-  read -cA words
-
-  if [ "${#words}" -eq 2 ]; then
-    completions="$(rbenv commands)"
-  else
-    completions="$(rbenv completions ${words[2,-2]})"
-  fi
-
-  reply=("${(ps:\n:)completions}")
-}