Move flex style's minibuffer-default-aware sorting to lisp/icomplete.el

Move flex style's minibuffer-default-aware sorting to lisp/icomplete.el

This moves the logic from the series of commits starting in the commit named:

  Improve sorting of flex completion style with non-nil minibuffer-default

to lisp/icomplete.el, so far the only confirmed beneficiary of that
functionality.

* lisp/icomplete.el (icomplete--sorted-completions): Consider
minibuffer-default here.

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