revert map pooling from #11484

[cstyan]

Since we're seeing panics related to this and we can't reproduce it in a test, safer to just roll the change back for now.

[slim-bean]

pools.....

[grafanabot]

Hello @cstyan!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[github-actions]

Trivy scan found the following vulnerabilities:

• HIGH, Target: docker.io/grafana/loki:main-599eed7 (alpine 3.18.4), Type: alpine openssl: Incorrect cipher key and IV length processing in libcrypto3 v3.1.3-r0. Fixed in v3.1.4-r0
• HIGH, Target: docker.io/grafana/loki:main-599eed7 (alpine 3.18.4), Type: alpine openssl: Incorrect cipher key and IV length processing in libssl3 v3.1.3-r0. Fixed in v3.1.4-r0
  \nTo see more details on these vulnerabilities, and how/where to fix them, please run docker build -t grafana/loki:main-599eed7 -f cmd/loki/Dockerfile .
  trivy i grafana/loki:main-599eed7 on your branch. If these were not introduced by your PR, please considering fixing them in via a subsequent PR. Thanks!

[cstyan]

closing, we've reverted this change completely in k183 and fixed properly in k184