[GOBBLIN-2140] Remove isMultiActiveSchedulerEnabled flag and its usages

[abhishekmjain]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/GOBBLIN-2140

Description

• Here are some details about my PR, including screenshots (if applicable):
  Removed isMultiActiveScheduler flag from:
  1. GobblinServiceGuiceModule and FlowLaunchHandler
  2. GobblinServiceManager and set scheduler to active for all hosts
  3. DagActionStoreChangeMonitor and its factories

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  1. Removed the test cases using this flag

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.27%. Comparing base (e501b62) to head (adaf0b5).
Report is 9 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4035      +/-   ##
============================================
+ Coverage     45.86%   50.27%   +4.41%     
- Complexity     3257     5906    +2649     
============================================
  Files           707     1073     +366     
  Lines         27865    41232   +13367     
  Branches       2796     4628    +1832     
============================================
+ Hits          12779    20728    +7949     
- Misses        14008    18728    +4720     
- Partials       1078     1776     +698     

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[phet]

this PR may be superseded by #4031

as that's been merged, please rebase and check whether other changes remain here