Remove unstable test

This test occasionally failed in CI. This test purported to test that
context watch doesn't leak goroutines. But goroutines can be created
at any time by the Go runtime. These goroutines could also finish at any
time.

To properly test for leaked goroutines, we would need to use something
like go.uber.org/goleak. But I'd rather not add a dependency.

The new context watch implementation doesn't manually create goroutines,
so we can be reasonably confident that it doesn't leak goroutines
without this test.

Author: jackc