Fix bug in flex completion style's sorting and simplify

Fix bug in flex completion style's sorting and simplify

This previous commit targetting this function introduced a bug whereby
the completion table's sorting function wouldn't be called.  That is
fixed by this commit, which also simplifies the function further: it
now skips re-sorting the completions completely if there is no
minibuffer input at all (in other words, when flex isn't doing
anything useful).

* lisp/minibuffer.el (completion--flex-adjust-metadata): Simplify.