chore(gulp/travis): don't build Dart docs

[chalin]

Contributes to #2930

cc @naomiblack @juleskremer

[chalin]

In this PR:

• All gulp tasks will now build for ts|js by default (it used to be ts|js|dart).
• I've pulled out the build safeguards that previously would force a build of Dart API docs on a check-deploy.
• Deleted the build scripts used to install Dart repo and tooling.
• Travis: removed reference to the deleted build scripts (only affected deploy-install.sh).
• Added an ignore rule to firebase.json so that docs/dart/** is ignored (even if present).

Note that Dart doc pages will still be generated by Harp (but those take a trivial amount of time to generate).

Travis is green.

cc @filipesilva @wardbell