It's been gone for years, and it's not coming back. If we support
extensions in the future, it would be via the WebExtensions API.

This makes me less annoyed by having to maintain a meaningless
distinction between installed and non-installed header files (it's been
years since any headers were installed). It also makes me less annoyed
by having to look at meaningless direct-inclusion guards. And finally,
it makes me less annoyed because each time I see those direct-inclusion
guards, I think of how they really ought to be inside the normal include
guards, rather than outside, to avoid breaking GCC and Clang's
optimization to avoid unnecessarily reading the contents of the header
file. (WebKit gets this wrong, too.) You can thank me if this makes
Epiphany compile 0.1s or so faster for you.

There are no useful changes in this commit.