Make controller's os.RemoveAll single-threaded

[Warashi]

What this PR does / why we need it:

This PR modifies the planner/scheduler cleanup to be single-threaded in order to reduce the file IO loads. I did a load test with the following scenario and discovered that os.RemoveAll creates a high load for piped:

• A single piped instance manages 400 apps
• All 400 apps are triggered by one commit

In this scenario, the piped requires about 20 minutes to complete all deployments. After this PR, the piped requires about 15 minutes to complete.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?: Yes

• How are users affected by this change:

  • The piped requires larger disk space.
  • The piped performance becomes better

• Is this breaking change:

• How to migrate (if breaking change):

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.

Project coverage is 22.81%. Comparing base (5b9d417) to head (efa4e6d). Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/piped/controller/controller.go	0.00%	18 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5217 +/- ## ========================================== - Coverage 22.83% 22.81% -0.03% ========================================== Files 420 420 Lines 45302 45316 +14 ========================================== - Hits 10344 10338 -6 - Misses 34163 34183 +20 Partials 795 795 ```

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