Remove `module-headings.php` from version control

[MichaelArestad]

module-headings.php is an automatically compiled file which occasionally results in some pretty weird stuff if people forget to build it when changing headers. Let's just remove it from git.

• We should probably move the built file to the build folder
• We should also add it's build to the watch task

[dereksmart]

pretty weird stuff

What kind of weird stuff?

Let's just remove it from git.

Let's not, it's nice to have in git as a reminder and also as visual verification that things are correct instead of blindly pushing it to svn or the built branches.

It's easy to create a PR with the changes whenever you see there's a diff in the file, and should be done when the headers have changed.