power: supply: pcf50633-charger: remove redundant variable charging_start

Variable charging_start is being set but is never read, it is therefore
redundant and can be removed. Cleans up sparse warning:

drivers/power/supply/pcf50633-charger.c:61:3: warning: Value stored to
'charging_start' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>