Re-add zsh completion script and remove fish completion script

Looks like the zsh completion script was removed by mistake.

Fixes: https://github.com/rbenv/rbenv/issues/1215
Refs: 569d464d36

completions/rbenv.zsh

@@ -0,0 +1,18 @@
+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}")
+}