completion: Unset local variable outside condition block
completion: Unset local variable outside condition block

Unset `curl_prefix` outside condition block so that it is always
cleared. Also, avoid `brew --prefix <foo>` since it is triggering ruby
in fallback flow when `<foo>` is not present.

Also, apply minor formatting tweaks.

randomizedthinking/prezto,imcvampire/prezto,ParthKolekar/prezto,tcbbd/prezto,louis-hsu/prezto,adamwalz/prezto,drpebcak/prezto,yyadavalli/prezto,pedroxs/prezto,wzxu/prezto,kepbod/prezto,sorin-ionescu/prezto,indrajitr/prezto,jamesacarr/prezto,brianhartsock/prezto,hghwng/prezto,tymcauley/prezto,donileo/prezto,ferronrsmith/prezto,jeffcox/prezto,archfear/prezto,vogler/prezto,mattratleph/prezto,nstCactus/prezto