Revert "Fix `MemPostings.Add` and `MemPostings.Get` data race (#15141)" #621

Job	Run time
Check generated parser	17s
Build Prometheus for all architectures	0s
fuzzing	0s
Build Prometheus for common architectures (0)	6m 50s
Build Prometheus for common architectures (1)	6m 41s
golangci-lint	3m 37s
Go tests	12m 27s
More Go tests	9m 12s
Build Prometheus for common architectures (2)	6m 37s
Go tests with previous Go version	7m 51s
Mixins tests	2m 18s
UI tests	2m 23s
Go tests on Windows	8m 44s
codeql / Analyze (javascript)	1m 46s
Report status of build Prometheus for all architectures	0s
Publish UI on npm Registry	1m 52s
Publish release artefacts	0s
Publish main branch artifacts	0s
	1h 10m 35s

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