Make description of text properties on the form `(string ...)' work

Make description of text properties on the form `(string ...)' work

* lisp/descr-text.el (describe-property-list): Don't special-case
for symbols that have widget properties here (bug#22957).  It's
not documented that this function should do that, and looking at
the code, it doesn't seem like this function is actually used for
doing that, either.  This makes describing some text properties
that are on the form `(string ...)' work.