Delete primary_extension from language data

The language attribute is still maintained as the first extension found.

This allows Mercury to be properly detected by Linguist, as per #748.
Merge branch 'master' into 985

Conflicts:
	lib/linguist/languages.yml
Note on extensions
Formatting
Formatting
Merge pull request #1167 from github/the-end-of-primary-extension

RIP `primary_extension`. / cc @nox @tnm @bkeepers @rick