Remove unnecessary String() calls with stringer interface

[sejongk]

What this PR does / why we need it:
This PR removes unnecessary String() calls with stringer interface.
We hope this style to be applied to the overall code base to keep it simple.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (0a7f4c8) 50.71% compared to head (3ad01f0) 50.71%.

Files	Patch %	Lines
server/backend/sync/memory/pubsub.go	0.00%	9 Missing ⚠️
server/backend/database/memory/database.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #777   +/-   ##
=======================================
  Coverage   50.71%   50.71%           
=======================================
  Files          70       70           
  Lines       10213    10213           
=======================================
  Hits         5180     5180           
  Misses       4512     4512           
  Partials      521      521           

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[hackerwins]

Thanks for your contribution.

[krapie]

LGTM 👍🏼