fix: Removed unused flushInterval from WriteApi

[rolincova]

Closes

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• make test completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #33 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #33      +/-   ##
============================================
- Coverage     85.71%   85.69%   -0.03%     
  Complexity      276      276              
============================================
  Files            15       15              
  Lines           630      629       -1     
============================================
- Hits            540      539       -1     
  Misses           90       90              

Impacted Files	Coverage Δ	Complexity Δ
src/InfluxDB2/Client.php	100.00% <ø> (ø)	6.00 <0.00> (ø)
src/InfluxDB2/WriteOptions.php	100.00% <ø> (ø)	1.00 <0.00> (ø)

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