Remove redundant SubArray tests

[timholy]

Now that we have only view rather than both sub and slice, we can cut the number of executed tests in subarray.jl nearly in half. Sadly, this has negligible effect on the test time, because the time is nearly 100% codegen (the infamous loop goes from >200s to under 1s if you run it a second time, as long as you don't redefine runsubarraytests and runviews).

Still, it seems worth deleting duplicated code.

[tkelman]

appveyor failure looks like #16091 (comment)