[fix](ann index)Remove redundant omp_set_num_threads

[uchenily]

No description provided.

[Thearas]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[uchenily]

run buildall

[doris-robot]

ClickBench: Total hot run time: 28.87 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit f42606d37d7218f39df8f8f7fd6c7675169b8fd1, data reload: false

query1	0.06	0.05	0.05
query2	0.10	0.05	0.05
query3	0.26	0.09	0.09
query4	1.61	0.11	0.12
query5	0.29	0.26	0.25
query6	1.18	0.66	0.66
query7	0.03	0.03	0.04
query8	0.07	0.05	0.05
query9	0.62	0.54	0.53
query10	0.60	0.60	0.59
query11	0.17	0.12	0.12
query12	0.16	0.12	0.12
query13	0.63	0.62	0.62
query14	1.04	1.05	1.05
query15	0.87	0.89	0.86
query16	0.42	0.42	0.44
query17	1.09	1.09	1.06
query18	0.22	0.21	0.21
query19	1.95	1.87	1.92
query20	0.02	0.02	0.02
query21	15.41	0.19	0.13
query22	5.04	0.07	0.05
query23	15.68	0.27	0.11
query24	2.87	0.38	0.49
query25	0.08	0.05	0.07
query26	0.15	0.14	0.14
query27	0.06	0.05	0.06
query28	3.95	1.15	0.94
query29	12.58	4.23	3.40
query30	0.28	0.15	0.12
query31	2.83	0.60	0.40
query32	3.25	0.55	0.50
query33	3.06	3.15	3.18
query34	16.20	5.48	4.88
query35	4.99	4.90	4.96
query36	0.70	0.52	0.52
query37	0.10	0.08	0.07
query38	0.07	0.05	0.04
query39	0.04	0.03	0.04
query40	0.18	0.15	0.16
query41	0.09	0.04	0.04
query42	0.05	0.03	0.03
query43	0.05	0.04	0.03
Total cold run time: 99.1 s
Total hot run time: 28.87 s

[doris-robot]

BE UT Coverage Report

Increment line coverage 100.00% (3/3) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.64% (17931/34061)
Line Coverage	37.88% (162663/429395)
Region Coverage	32.27% (123998/384273)
Branch Coverage	33.69% (54356/161318)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (3/3) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.45% (23864/33398)
Line Coverage	57.82% (248158/429215)
Region Coverage	52.85% (205717/389282)
Branch Coverage	54.58% (88558/162239)

[github-actions]

PR approved by anyone and no changes requested.

[airborne12]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.