pm2301-charger: Removed unused code from PM2301 driver

Some of the headers and defines accrued over time are no longer
in use. Let's take the opportunity to remove a few of them.

Signed-off-by: Rupesh Kumar <rupesh.kumar@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Marcus COOPER <marcus.xm.cooper@stericsson.com>
Reviewed-by: Philippe LANGLAIS <philippe.langlais@stericsson.com>