pip: fix smoke test double-TensorBoard dependency

Summary:
The smoke test doesn’t actually catch those things that it’s supposed to
catch, because it installs `tf-nightly` (or `tf-nightly-2.0-preview`),
which in turn installs `tb-nightly`. We then install the just-built
version of `tensorboard`, so the resulting virtualenv has both
`tensorboard` and `tb-nightly`: #1907 strikes again.

When `--tf-version ""` is passed, this is not an issue, but some smoke
checks only run when that is not passed.

It just so happens that this is currently benign: each of the direct
dependencies of `:build_pip_package` either has no smoke test, contains
an `__init__.py` file that is required to overwrite the Bazel-generated
empty one, because the smoke test references symbols directly in that
file, or is not yet in `tb-nightly`. But (a) starting with tomorrow’s
nightly, the mesh smoke test will meet none of those conditions, so its
smoke test will cease to be effective, and (b) I’d like to add an
hparams smoke test, which already falls into the same boat.

Test Plan:
Add an hparams smoke test, but remove the actual hparams dep:

```diff
diff --git a/tensorboard/pip_package/BUILD b/tensorboard/pip_package/BUILD
index c11e22e2..e63b790a 100644
--- a/tensorboard/pip_package/BUILD
+++ b/tensorboard/pip_package/BUILD
@@ -21,7 +21,6 @@ sh_binary(
         "//tensorboard:lib",  # User-facing overall TensorBoard API
         "//tensorboard:version",  # Version module (read by setup.py)
         "//tensorboard/plugins/beholder",  # User-facing beholder API
-        "//tensorboard/plugins/hparams",  # User-facing hparams API
         "//tensorboard/plugins/mesh:summary",  #  User-facing mesh summary API
         "//tensorboard/plugins/projector",  # User-facing projector API
         "//tensorboard/summary:tf_summary",  #  tf.summary API for TF 2.0
diff --git a/tensorboard/pip_package/build_pip_package.sh b/tensorboard/pip_package/build_pip_package.sh
index de778196..113724ef 100755
--- a/tensorboard/pip_package/build_pip_package.sh
+++ b/tensorboard/pip_package/build_pip_package.sh
@@ -172,6 +172,7 @@ import tensorboard as tb
 tb.summary.scalar_pb('test', 42)
 from tensorboard.plugins.beholder import Beholder, BeholderHook
 from tensorboard.plugins.mesh import summary
+from tensorboard.plugins.hparams.api import hparams_pb, KerasCallback
 "
   fi

```

Then run

    bazel run //tensorboard/pip_package:build_pip_package

and note that the smoke test passes (incorrectly) before this commit and
fails (correctly) after it.

Note also that the `pip freeze` output from the smoke test includes both
`tb-nightly` and `tensorboard` before this change (e.g., on recent
Travis logs), but only `tensorboard` after it.

wchargin-branch: pip-smoke-2dep

Author: wchargin

tensorboard/pip_package/build_pip_package.sh

@@ -142,6 +142,7 @@ smoke() {
   if [ -n "${smoke_tf}" ]; then
     pip install -qU "${smoke_tf}"
   fi
+  pip uninstall -qy tensorboard tb-nightly  # Drop any conflicting packages
   pip install -qU ../dist/*"py${py_major_version}"*.whl >/dev/null
   pip freeze  # Log the results of pip installation