Add command project-remove-known-project

Add command project-remove-known-project

* etc/NEWS: Mention the new command.

* lisp/progmodes/project.el (project--remove-from-project-list): Add
new argument, report-message, used to signal various messages when
removal has happened.

* lisp/progmodes/project.el (project-remove-known-project): New
command that removes the selected directory from the project-list-file.

* lisp/progmodes/project.el (project-current): Add the report message.

* doc/emacs/maintaining.text: Add information about the new command to
the manual.