power: supply: ab8500: Drop unused battery types

The code tries to detect a lot of battery variants on the reference
designs, but we are not using the reference designs in practice, we
are using real products such as Samsung Phones.

The reference design with no battery plugged in will be detected as
a LIPO battery with a thermistor on the batctrl pin so we will
assume this and later on we can support other types through the
device tree if we want, just like the products do.

Drop the tables for external thermistor, only keep the internal
thermistor tables that we will use as default.

We can delete the assignment of the temperature to resistance table
since the default will be the only and correct option.

Also get rid of some unused variables and unused exports.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>