Rename `clojure-docstring-fill-prefix' to `clojure-docstring-fill-prefix-width'

* clojure-mode.el (clojure-docstring-fill-prefix-width): Rename
from `clojure-docstring-fill-prefix'.
(clojure-docstring-fill-prefix-string): Remove.
(clojure-mode, clojure-adaptive-fill-function)
(clojure-fill-paragraph, clojure-indent-line): Don't use
`clojure-docstring-fill-prefix-string'.