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>