runtime: remove VDSO fallback test and benchmarks

These tests assume that it is OK to switch between time implementations,
but the clock_gettime call uses CLOCK_MONOTONIC and the fallback call,
gettimeofday, uses CLOCK_REALTIME. Disabling the clock_gettime call means
that calls to nanotime will start returning very different values.
This breaks the new timer code, which assumes that nanotime will return
a consistently increasing value.

This test is not very useful in any case as it doesn't check the results.

Removing this file also removes BenchmarkTimeNow, which is a duplicate
of BenchmarkNow in the time package.

Updates golang#27707
Fixes golang#32109

Change-Id: I6a884af07f75822d724193c5eed94742f524f07d
Reviewed-on: https://go-review.googlesource.com/c/go/+/174679
Reviewed-by: Brad Fitzpatrick <[email protected]>