Remove Travis configuration from flutter/flutter (#20288)

This removes the final traces of Travis and Appveyor from the Flutter tree.

I've updated the documentation and fixed a couple of places where scripts look for Travis, and eliminated the dart tools runningOnTravis function (which was unused anyhow).

There are places in the flutter script that used to look for the environment variable TRAVIS. We actually do want to continue to detect that we're running on Travis there, since in the plugins repo we still use Travis (for the moment). In any case, it's OK, because the CI environment variable is set on all of the CI bots (Cirrus, Travis, and Appveyor).

FastLane doesn't have a setup_cirrus equivalent to setup_travis, but it actually doesn't matter there either, since it doesn't do Travis-specific things, and it also looks for the CI environment variable.

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 5621cdb..78388f9 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -17,7 +17,7 @@
 # * stage: one of "devicelab", "devicelab_ios", "devicelab_win"
 #       tasks are grouped by stage so they appear next to each on the dashboard and have a distinct
 #       icon attached to them. Stages can be used to create dependencies, e.g. we may decide to not
-#       run device tests on red Travis.
+#       run device tests on red Cirrus builds.
 # * required_agent_capabilities: a list of strings
 #       list of capabilities a devicelab agent must have to be able to run this task.
 #