Cleanup startPlayingFile

[TheNitek]

startPlayingFile seems overly complicated right now, so I'd suggest to to clean it up a bit. While filling the buffer initially sounds nice, it doesn't seem to bring any real benefit (you have to start refilling right away anyway).
Right now it's very confusing:

• feedBuffer() is called within a loop that is happening within feedBuffer_noLock anyway
• interrupts() is called afterwards even though interrupts are already enabled by feedBuffer()

In my opinion removing the complexity outweighs the "need" to initially fill the buffer.

[johnwargo]

@TheNitek this PR is 5 years old now and nothing's happened with it. Close it?