Remove Flush operation before create index

[congqixia]

See also #594
sync behavior with pymilvus
/kind improvement

[sre-ci-robot]

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[codecov]

Codecov Report

Merging #598 (6503c24) into master (b1c9ccb) will decrease coverage by 0.07%.
Report is 4 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #598      +/-   ##
==========================================
- Coverage   82.92%   82.85%   -0.07%     
==========================================
  Files          40       40              
  Lines        3643     3640       -3     
==========================================
- Hits         3021     3016       -5     
- Misses        511      512       +1     
- Partials      111      112       +1     

Files	Coverage Δ
client/index.go	77.60% <ø> (-0.53%)	⬇️

... and 1 file with indirect coverage changes

[yah01]

/lgtm