build: decrease karma timeouts for local browsers

[devversion]

Currently the timeouts which have been adjusted for a
flaky browser connection are also used for local browser
runs. This is problematic as local browsers which do
not capture properly still wait multiple minutes, while the
browser can just be restarted without hitting any rate
limit.

Additionally removes the custom FirefoxHeadless browser
because the karma-firefox-launcher also provides the
FirefoxHeadless browser configuration.

[andrewseguin]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: api_golden_checks" is failing
     status "ci/circleci: bazel_build_test" is failing
     status "ci/circleci: build_release_packages" is failing
     status "ci/circleci: e2e_tests" is failing
     status "ci/circleci: lint" is failing
     status "ci/circleci: prerender_build" is failing
     status "ci/circleci: tests_browserstack" is failing
     status "ci/circleci: tests_local_browsers" is failing
     status "ci/circleci: tests_saucelabs" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

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