Remove link to outdated multiapi generation script

[mccoyp]

Description

#41943 removed multiapi_init_gen.py, breaking the link in this doc. @msyyc, let me know if we should more significantly overhaul this multiapi.md or remove it altogether.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull Request Overview

This PR removes the broken link to the now-deleted multiapi_init_gen.py script from the multiapi documentation.

• Deleted the outdated reference to multiapi_init_gen.py
• No functional code changes

Comments suppressed due to low confidence (1)

doc/dev/mgmt/multiapi.md:106

• [nitpick] Grammatical error: 'There is as many folder per API versions we want to ship' should be 'There are as many folders as API versions we want to ship.'

- There is as many folder per API versions we want to ship

[msyyc]

@mccoyp Thanks for your PR. multiapi feature in codegen will be deprecated in several months but now it is still in use so we had better keep this doc until we drop the feature.