revert: "fix: KVSearchDriver is_merge should be False by default"

[hanxiao]

Reverts #1701 due to #1705

[codecov]

Codecov Report

Merging #1706 (8fc8a14) into master (da2be57) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1706   +/-   ##
=======================================
  Coverage   84.78%   84.78%           
=======================================
  Files         133      133           
  Lines        6856     6856           
=======================================
  Hits         5813     5813           
  Misses       1043     1043           

Impacted Files	Coverage Δ
jina/drivers/search.py	96.49% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c56f348...342d1da. Read the comment docs.

[github-actions]

Latency summary

Current PR yields:

• 😶 index QPS at 1605, delta to last 3 avg.: +0%
• 😶 query QPS at 28, delta to last 3 avg.: -1%

Breakdown

Version	Index QPS	Query QPS
current	1605	28
0.9.15	1594	28

Backed by latency-tracking. Further commits will update this comment.