fix: hide workplace suggest on Sources placeholder element

[hoxyq]

Summary

Continuation of 8fa79ca. Overlooked while upgrading the fork in #172.

Test plan

I've also double-checked that already gated things from 8fa79ca are still present in the code and are not available in the UI, like Workplace tabs.

Before	After

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.