Properly detect Zsh shell

There is no need to define custom shell var, when Zsh can be detected by
checking SHELL env var.

autocomplete/zsh_autocomplete

@@ -4,9 +4,9 @@ local -a opts
 local cur
 cur=${words[-1]}
 if [[ "$cur" == "-"* ]]; then
-  opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} ${cur} --generate-bash-completion)}")
+  opts=("${(@f)$(${words[@]:0:#words[@]-1} ${cur} --generate-bash-completion)}")
 else
-  opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} --generate-bash-completion)}")
+  opts=("${(@f)$(${words[@]:0:#words[@]-1} --generate-bash-completion)}")
 fi
 
 if [[ "${opts[1]}" != "" ]]; then