Remove cancellationToken.js

[jakebailey]

cancellationToken.js is a file which contains an implementation of a file-based cancellation token. It is loaded dynamically by tsserver.js, but nowhere else. As far as I can tell, there's no reason to not just include its code in tsserver.js directly and drop the extra file and require.

(Going through the repo history also doesn't indicate why this was a separate file; perhaps there's a good reason for this to exist and I lack context.)