My Jetpack: Show in multisite installations

[oskosk]

Fixes #35555

Proposed changes:

• Removes the check for multisite that was making My Jetpack not load in subsites of multisite installations.

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

No

Testing instructions

• Launch a multisite. Create a subsite
• Network activate Jetpack
  • Expect to see the error reported in #35557 but Jetpack will be network activated
• Visit the subsite.
• Activate Jetpack
• Expect to be redirected to My Jetpack
• Expect to see the My Jetpack menu item