Clean up units

[dbrgn]

• Remove deprecated hPA unit from barometer sensor
• Remove redundant mW unit from power_consumption sensor
• Clean up nonsensical unit descriptions

There are other redundant units, for example temperature in kelvin or fahrenheit (and many others), or wind speed in m/s vs km/h. However, I decided not to touch those for now.

[s3lph]

I had a quick look through all the endpoints in the directory:

• The mW unit isn't used anywhere.
• The hPA unit is still used in 3 endpoints, but it's been deprecated long enough to warrant removal.

[dbrgn]

@s3lph great, thanks for the check!