[GetTransaction] remove unneeded cs_main lock acquire

Summary: Backport of core#22609.

Test Plan:
With Debug and Clang:

ninja all check-all

Run the TSAN build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12210