refactor(storage): remove unused FlushIo variant

[etolbakov]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Removed unused FlushIo variant .

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

#855

[etolbakov]

@evenyag
As per your suggestion in #855 FlushIo is removed.
Could you please have look in the meantime?

[codecov]

Codecov Report

Merging #1002 (0765abd) into develop (374acc8) will decrease coverage by 0.26%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1002      +/-   ##
===========================================
- Coverage    86.24%   85.98%   -0.26%     
===========================================
  Files          432      432              
  Lines        62494    62494              
===========================================
- Hits         53897    53736     -161     
- Misses        8597     8758     +161     

Flag	Coverage Δ
rust	85.98% <ø> (-0.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/object-store/src/test_util.rs	0.00% <0.00%> (-100.00%)	⬇️
src/common/substrait/src/df_expr.rs	30.81% <0.00%> (-16.56%)	⬇️
src/datanode/src/instance.rs	41.56% <0.00%> (-14.91%)	⬇️
tests-integration/src/test_util.rs	79.47% <0.00%> (-9.33%)	⬇️
src/datatypes/src/data_type.rs	98.26% <0.00%> (+0.34%)	⬆️
src/mito/src/table.rs	88.63% <0.00%> (+0.51%)	⬆️
src/common/substrait/src/df_logical.rs	78.77% <0.00%> (+0.87%)	⬆️
src/common/substrait/src/types.rs	51.92% <0.00%> (+0.96%)	⬆️
src/meta-srv/src/service/heartbeat.rs	87.23% <0.00%> (+1.06%)	⬆️
src/servers/src/opentsdb.rs	98.41% <0.00%> (+1.58%)	⬆️
... and 1 more

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[evenyag]

LGTM

[waynexia]

👍