Remove `--detach` parameter from .desktop files

Our .desktop files include the --detach parameter.

Looking at https://specifications.freedesktop.org/desktop-entry-spec/latest/exec-variables.html it doesn't look like it is useful to have the program detach from the starting process.

The --detach parameter can cause issues in some cases. See https://github.com/mullvad/mullvad-browser/issues/354.

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