This reverts commit 06741ceb62.
A couple people on Fedora 40 reported that the desktop file was no
longer working for them after this change, and it's not clear that
anyone was having trouble on Ubuntu before it -- someone just noted that
`desktop-file-validate` was showing some warnings.
Set MOZ_LEGACY_PROFILES environment variable to disable
dedicated-profile support, which creates dedicated profiles for
different channels (e.g., `esr`).
