Draft: Remove npm_allow_packages_in_multiple_projects feature flag

What does this MR do and why?

This rolls out the bugfix in !111775 (merged) which was behind the npm_allow_packages_in_multiple_projects feature flag.

Feature Flag Rollout issue: #391692

Screenshots or screen recordings

Results of testing on staging.

With npm_allow_packages_in_multiple_projects disabled:

rad@mjolnirv3 ~/p/t/foo> npm i @radsgroup/foo@2.0.0

added 1 package, removed 2 packages, and audited 2 packages in 2s

found 0 vulnerabilities
rad@mjolnirv3 ~/p/t/foo> npm i @radsgroup/foo@1.0.0
npm ERR! code ETARGET
npm ERR! notarget No matching version found for @radsgroup/foo@1.0.0.
npm ERR! notarget In most cases you or one of your dependencies are requesting
npm ERR! notarget a package version that doesn't exist.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/rad/.npm/_logs/2023-03-06T06_56_05_820Z-debug-0.log

foo@2.0.0 installs, foo@1.0.0 fails as expected.

With feature flag enabled:

rad@mjolnirv3 ~/p/t/foo> npm i @radsgroup/foo@2.0.0

up to date, audited 2 packages in 3s

found 0 vulnerabilities
rad@mjolnirv3 ~/p/t/foo> npm i @radsgroup/foo@1.0.0

changed 1 package, and audited 2 packages in 3s

found 0 vulnerabilities
rad@mjolnirv3 ~/p/t/foo> 

Both package versions install successfully.

How to set up and validate locally

Note: These have already been setup in the staging environment

1. Setup

• create a group named radsgroup
• create a project named project1 under radsgroup with public visibility
• create a project named project2 under radsgroup with public visibility
• publish an npm package named @radsgroup/foo version 1.0.0 under project1
• publish an npm package named @radsgroup/foo version 2.0.0 under project2
• create a personal access token
• add the instance endpoint with the personal access token to .npmrc

2. Test

npm i @radsgroup/foo@2.0.0 
npm i @radsgroup/foo@1.0.0

Both packages should install successfully.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #391692