ci: remove Travis configs (#4435)

Summary:
Some time between 13:41 and 17:20 Pacific time today, Travis stopped
even pretending to run our builds. Note: #4433 reports a Travis status,
whereas #4434 has no such item, not even “pending”. And our Travis
dashboard shows no builds more recent than #4433, either.

Relevant Hacker News: <https://news.ycombinator.com/item?id=25338983>

Thankfully, we finally landed #2953 on Friday, so we are no longer
dependent on Travis. This patch deletes the Travis configs and excises
all remaining mentions.

Test Plan:
Running `git grep -i travis` now fails.

wchargin-branch: ci-rm-travis

Author: wchargin

.travis.yml

@@ -1,4 +1,4 @@
-# TensorBoard [![Travis build status](https://travis-ci.org/tensorflow/tensorboard.svg?branch=master)](https://travis-ci.org/tensorflow/tensorboard/) [![GitHub Actions CI](https://github.com/tensorflow/tensorboard/workflows/CI/badge.svg)](https://github.com/tensorflow/tensorboard/actions?query=workflow%3ACI+branch%3Amaster+event%3Apush) [![Compat check PyPI](https://python-compatibility-tools.appspot.com/one_badge_image?package=tensorboard)](https://python-compatibility-tools.appspot.com/one_badge_target?package=tensorboard)
+# TensorBoard [![GitHub Actions CI](https://github.com/tensorflow/tensorboard/workflows/CI/badge.svg)](https://github.com/tensorflow/tensorboard/actions?query=workflow%3ACI+branch%3Amaster+event%3Apush) [![Compat check PyPI](https://python-compatibility-tools.appspot.com/one_badge_image?package=tensorboard)](https://python-compatibility-tools.appspot.com/one_badge_target?package=tensorboard)
 
 TensorBoard is a suite of web applications for inspecting and understanding your
 TensorFlow runs and graphs.

README.md

@@ -88,7 +88,7 @@ tf_workspace()
 
 load("@bazel_skylib//lib:versions.bzl", "versions")
 # Keep this version in sync with the BAZEL environment variable defined
-# in our .travis.yml and .github/workflows/ci.yml configs.
+# in our .github/workflows/ci.yml config.
 versions.check(minimum_bazel_version = "3.7.0")
 
 load("@io_bazel_rules_sass//:package.bzl", "rules_sass_dependencies")

WORKSPACE

@@ -44,7 +44,7 @@ check_urls_resolve() {
     exclude_buildozer=':!ci/download_buildozer.sh'  # likewise
     # We use `git-grep` to efficiently get an initial result set, then
     # filter it down with GNU `grep` separately, because `git-grep` only
-    # learned `-o` in Git v2.19; Travis uses v2.15.1.
+    # learned `-o` in Git v2.19.
     unresolved_urls_file="${tmpdir}/unresolved_urls"
     git grep -Ph "${url_pcre}" "${exclude_bazel}" "${exclude_buildifier}" \
         "${exclude_buildozer}" \