Remove special casing for duplicate gutter class

Atom removed the duplicate gutter class in ba21f0b0d8677583b33baa00d8282b3,
so there's no need to special case things any more.

Test Plan:
Opened git-blame in 0.117.0 and it worked
Merge pull request #15 from dmnd/remove-react-special-case

Remove special casing for duplicate gutter class