Remove database migration tool Flyway (#4353)

[nisiyong]

What's the purpose of this PR

Related #4353

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #4361 (b657806) into master (66374a1) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4361      +/-   ##
============================================
+ Coverage     53.40%   53.41%   +0.01%     
+ Complexity     2688     2687       -1     
============================================
  Files           490      490              
  Lines         15254    15254              
  Branches       1586     1586              
============================================
+ Hits           8146     8148       +2     
+ Misses         6552     6549       -3     
- Partials        556      557       +1     

Impacted Files	Coverage Δ
...work/apollo/biz/message/ReleaseMessageScanner.java	82.89% <0.00%> (-2.64%)	⬇️
.../apollo/internals/RemoteConfigLongPollService.java	77.24% <0.00%> (-1.20%)	⬇️
...ervice/service/ReleaseMessageServiceWithCache.java	85.88% <0.00%> (-1.18%)	⬇️
...nfigservice/service/AccessKeyServiceWithCache.java	84.69% <0.00%> (+2.04%)	⬆️
...ework/apollo/internals/RemoteConfigRepository.java	88.34% <0.00%> (+3.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 66374a1...b657806. Read the comment docs.

[nobodyiam]

@nisiyong As this is a breaking change for flyway users, I think it's better to update the CHANGES.md to reflect this change.

[nisiyong]

Okay

[nobodyiam]

LGTM