[GH-1633] Remove WFL_SLACK_ENABLED feature switch

[okotsopoulos]

Purpose

• https://broadinstitute.atlassian.net/browse/GH-1633

Feature switch originally added here: #571

Slack notifications have been verified to work in WFL prod.

I've also removed WFL_SLACK_ENABLED environment variable assignments in gotc-deploy repo: https://github.com/broadinstitute/gotc-deploy/pull/331

System Tests

Off of a local WFL without WFL_SLACK_ENABLED environment set, ran system tests and observed that the expected Slack message was emitted.

wm111-e35:wfl okotsopo$ cd api; WFL_WFL_URL=http://localhost:3000 clojure -M:test --focus wfl.system.v1-endpoint-test/test-workload-sink-outputs-to-tdr
--- system (clojure.test) ---------------------------
wfl.system.v1-endpoint-test
  test-workload-sink-outputs-to-tdr

1 tests, 7 assertions, 0 failures.

Top 1 slowest kaocha.type/clojure.test (769.46063 seconds, 100.0% of total time)
  system
    769.46063 seconds average (769.46063 seconds / 1 tests)

Top 1 slowest kaocha.type/ns (769.45496 seconds, 100.0% of total time)
  wfl.system.v1-endpoint-test
    769.45496 seconds average (769.45496 seconds / 1 tests)

Top 1 slowest kaocha.type/var (769.45032 seconds, 100.0% of total time)
  wfl.system.v1-endpoint-test/test-workload-sink-outputs-to-tdr
    769.45032 seconds wfl/system/v1_endpoint_test.clj:412

Full suite passes:

wm111-e35:wfl okotsopo$ make TARGET=system
export CPCACHE=/Users/okotsopo/wfl/api/.cpcache;            \
	export WFL_WFL_URL=http://localhost:3000; \
	clojure  -M:parallel-test wfl.system.v1-endpoint-test | \
	tee /Users/okotsopo/wfl/derived/api/system.log
WARNING: Specified path is external to project: ../derived/api/src
WARNING: Specified path is external to project: ../derived/api/resources

Ran 33 tests containing 374 assertions.
0 failures, 0 errors.
api system finished on Fri Mar 4 15:36:41 EST 2022
docs system finished on Fri Mar 4 15:36:41 EST 2022
functions/aou system finished on Fri Mar 4 15:36:41 EST 2022
functions/sg system finished on Fri Mar 4 15:36:41 EST 2022
helm system finished on Fri Mar 4 15:36:41 EST 2022
ui system finished on Fri Mar 4 15:36:41 EST 2022