Drop cleanupLegacySystemdAutostart, cleanupLegacyDesktopAutostart,
legacySystemdAutostartExists, legacyDesktopAutostartExists, and their
path helpers; remove calls from SetAutostart and AutostartStatus; drop
the os/exec import. Remove TestLinuxAutostartRemovesLegacyDesktopEntry
which referenced the deleted legacyAutostartDesktopPath function.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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