chore: remove unnecessary await

[vasild]

transaction pool's flush() method is not async, no need to call it with
await.

Proposed changes

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improve a current implementation without adding a new feature or fixing a bug)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build (changes that affect the build system)
• Docs (documentation only changes)
• Test (adding missing tests or fixing existing tests)
• Other... Please describe:

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov-io]

Codecov Report

Merging #1559 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1559   +/-   ##
========================================
  Coverage    74.75%   74.75%           
========================================
  Files          439      439           
  Lines         7894     7894           
  Branches      1038     1038           
========================================
  Hits          5901     5901           
  Misses        1770     1770           
  Partials       223      223

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