Simplify cancel() method

[ehmicky]

This is a follow up on #189 by @ammarbinfaisal. It simplifies the cancel() method by removing some code, without changing behavior.

[sindresorhus]

Are you sure these checks are not needed? Could be that we're just not having tests for their behavior.

[ehmicky]

Yes I should have provided some background information on the changes:

• isCanceled is always false because if cancel() was called, then result.signal will be defined.
• spawned.kill() is a fast noop (does not throw and returns false) if the child process has been killed. I.e. checking if (spawned.killed) {} before is not needed.