span: clean up test code after llrb frontier deletion

This patch cleans up some code in the unit tests that previously
would toggle the frontier type but now just leads to each test
being running twice with the exact same configuration.

Release note: None

Author: andyyang890

pkg/util/span/BUILD.bazel

@@ -34,7 +34,6 @@ go_test(
     embed = [":span"],
     deps = [
         "//pkg/roachpb",
-        "//pkg/testutils",
         "//pkg/util/hlc",
         "//pkg/util/leaktest",
         "//pkg/util/log",