Remove `brackets.app.abortQuit` check

[le717]

I found this while adding the project name in the window title.

src/document/DocumentCommandHandlers.js, lines 1283-1286 and 1314-1317:

// TODO: remove this if statement when we move to the new CEF3 shell
if (brackets.app.abortQuit) {
    brackets.app.abortQuit();
}

These checks were added two years ago (a61fb69). I think they can be removed now.