Remove a known project from the project-list-file.
Normally Emacs automatically adds and removes projects to and from the
project-list-file, but sometimes you may want to manually edit
the available projects. M-x project-forget-project
prompts you to choose one of the available projects, and then removes
it from the file.
The user option project-list-exclude may be set to always
ignore certain projects from being remembered and saved to
project-list-file. It is a list of regular expressions and
predicate functions for project roots and objects. The regexp is
matched against the project root, and the predicate should take the
project object as the only argument and return non-nil if the
project should not be saved to project-list-file.