Serial: Only use workaround if we're using serialEvent
Serial: Remove debug code again
Serial: Revert jSSC to the version that was shipped with 2.1

This removes some debug code that was temporarily added and turns yield() back into a 100ns delay, which probably won't make a difference for throughput but will be a bit lighter on the CPU.
Merge pull request #2265 from gohai/serial-new

Some cleanups for Serial