Remove snark key download

[mrmr1993]

trafficstars

We no longer store snark keys in S3, so this codepath is entirely unused. This PR removes the compile-time configuration option and the logic for downloading those keys.

[mrmr1993]

!ci-build-me

[martyall]

#15837 subsumes a lot of the changes to mlh files, but doesn't eliminate the feature. perhaps we could look at merging that and then removal ?