configs: remove CONFIG_SUN50I_A64_UNSTABLE_TIMER
    
    The patch that added the Kconfig symbol SUN50I_A64_UNSTABLE_TIMER was
    dropped in commit 60a8ce36abae7 ("Raspberry Pi DT updates, Update
    AllWinner A64 timer errata workaround"). So it's safe to drop it from
    the configuration generation system too.
    
    Signed-off-by: Paul Bolle <pebolle@tiscali.nl>