Make ‘delete-trailing-whitespace’ delete spaces after form feed

Make ‘delete-trailing-whitespace’ delete spaces after form feed

* lisp/simple.el (delete-trailing-whitespace): Treat form fead as
a non-whitespace character (regradless of whether it’s character syntax
is whitespace) and delete any whitespace following it instead of leaving
lines with form feeds completely unchanged.  I.e. a line like "\f " will
now became "\f".