It doesn't seem to have had any useful purpose back when the file was
added in bug 792188, and it sure doesn't have any right now, except
subtly breaking things by removing any `inline` from system headers,
with consequences on the definition of what should be COMDATs,
ultimately leading to link failure.

This is, in fact, very similar to bug 918943, which hid the #define in
some configurations.

Differential Revision: https://phabricator.services.mozilla.com/D177710