[2.6] Remove possible call_user_func()
minor #12532 [2.6] Remove possible call_user_func() (nicolas-grekas)

This PR was merged into the 2.6 branch.

Discussion
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[2.6] Remove possible call_user_func()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Remove possible call_user_func() introduced in 2.6

Commits
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1de4920 [2.6] Remove possible call_user_func()