Removing interleaved mode

[Hal Murray]

dfoxfranke at gmail.com said:
> With Eric's permission, I have removed support for interleaved mode in my
> proto-refactor branch. Here is its commit-message eulogy: 

Seems fine with me.  I've never used it.

We should test things to make sure nothing strange happens.  I think that 
requires 4 systems: 2 new and 2 old.  (I guess it could be done with 2 if you 
are willing to run the tests serially.)

This might be a good candidate for Pis with GPS HATs.  It might take the 
kernel PLL to notice the difference.

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