chore(docs-app): remove obsolete directives

Most of the directives in bootstrap.js haven't been in use since
https://github.com/angular/angular.js/commit/389d4879da4aa620ee95d789b19ff9be44eb730a:
Dropdown-related directives were moved to dropdown-toggle.js, and for
foldout, popover and syntax, the uses and tests were removed, but not the directives themselves.

The last use of tabbable was removed in
https://github.com/angular/angular.js/commit/6b7a1b82bc26bbf4640506a9a3cf37ebf254d3d2