.org Plans: Use site.domain instead of slugToUrl helper

[tyxla]

Reverts #9207 and uses site.domain instead of the slugToUrl helper that was previously introduced. Props to @mtias for noticing this.

There should be no observable changes or regressions caused by this PR.

To test:

• Checkout this branch
• Purchase a plan for a Jetpack site which is installed in a subdirectory
• Go through the process of canceling the plan
• Verify the site slug is displayed with / and not with :: in the "Are you sure you want to remove {PLAN} from {SLUG}" text in the removal step
• Verify the site slug is displayed with / and not with :: in the "{PLAN} was removed from {SLUG}" text in the removal confirmation notice.

/cc @beaulebens @johnHackworth @mtias

[matticbot]

Test live: https://calypso.live/?branch=update/use-domain-instead-of-slugtourl

[beaulebens]

This tests nicely for me. Confirmed that the URLs show up properly.

[mtias]

Thanks! Looks good to me once you fix the merge conflict.