power: supply: twl4030-charger: remove nonstandard max_current sysfs attribute

Since we now support the standard 'input_current_limit' property by

commit 3fb319c2 ("power: supply: twl4030-charger: add writable INPUT_CURRENT_LIMIT property")

we can now remove the nonstandard 'max_current' sysfs attribute.

See Documentation/power/power_supply_class.txt line 125

Both are functionally equivalent. From ABI point of view it is just a rename
of the property.

This also removes the entry in Documentation/ABI/testing/sysfs-class-power-twl4030

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>