release-23.2: sql/license: Fix data race in license/enforcer.go

[blathers-crl]

Backport 1/1 commits from #130036 on behalf of @spilchen.

/cc @cockroachdb/release

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There is a data race in TestQueryCache related to license/enforcer.go. This test is unique because it runs parallel tests that start a mock server, going through the SQL startup code. Since the license enforcer is a singleton, each parallel goroutine shares the same enforcer object. This commit addresses two issues:

1. The Enforcer.db variable can be set twice and is otherwise unprotected. Since the db is only needed during setup, I’ve removed the variable from the struct for now. I may need to reintroduce it later, but I’ll ensure it doesn’t break this test when I do.

2. The creation of the singleton in GetEnforcerInstance can cause a data race on the instance variable. While there is a once.Do call to generate the singleton, there was an unnecessary instance == nil check before it. I’ve removed this check to prevent the data race.

Epic: CRDB-39988
Closes: #130034
Release note: None

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Release justification: This work is part of the CockroachDB core deprecation.

[blathers-crl]

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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is