fix: properly free remote objects

[nornagon]

Description of Change

This was introduced when switching to mojo; at one point when the mojo change was still in progress the browser interface was an associated interface, but it no longer is, so this was failing silently (!).

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed an issue where objects referenced by remote could sometimes not be correctly freed.

[release-clerk]

Release Notes Persisted

Fixed an issue where objects referenced by remote could sometimes not be correctly freed.

[trop]

I was unable to backport this PR to "6-1-x" cleanly;
you will need to perform this backport manually.

[trop]

I was unable to backport this PR to "7-0-x" cleanly;
you will need to perform this backport manually.

[trop]

@nornagon has manually backported this PR to "7-0-x", please check out #20693

[trop]

@nornagon has manually backported this PR to "6-1-x", please check out #20694