Cleanup old battery management source files, not relevant to M5 Core2

[louisbertrand]

The file battery.cpp that defines the class BatteryMonitorIP5306 is no longer needed because M5 Core2 uses the AXP192 power management IC instead of the IP5306 the original M5 Core.
The Core2 library defines its own class AXP192, and there is an instance available to use in the m5 class. It builds, but may issue warnings about battery.cpp.
TODO: eventually simplify and remove battery.* from the build.