Removed ZHA units and unit validation

[cfulton]

This is an alternative to PR #331

Breaking Change:

• Any quirks that use the ZHA units will need to be updated to use a string value of the unit or to use the homeassistant const units enums instead.

This PR:

• Removed unit definitions from ZHA
• Updated references to units from HA instead
• Removed validate units on quirks

Should resolve the following issues:

• Sensor init fails if unit of measurement is a string #326
• validate_unit doesn't allow valid values #327
• [Device Support Request] Tuya ultrasonic level sensor (TS0601 by _TZE284_kyyu8rbj) zha-device-handlers#3356
• Fix missing unit in _TZE200_pay2byax quirk zha-device-handlers#3563
• Add HA entities for Schneider Electric Thermostats zha-device-handlers#3575

[cfulton]

a0e2b93

This commit is in case we do not want to have a dependency on home assistant.

[TheJulianJES]

We do not want to completely remove units. The issue is that they're duplicated in both quirks v2 in zigpy and in ZHA right now.