Remove translation_key for Tuya quirk v2 thermostat

[TheJulianJES]

Proposed change

Removes translation_key for Tuya v2 thermostat quirk, as HA should use the default device class names for these.
Also adjusted the fallback_name (default string in auto-generated HA strings.json) to "Thermostat mode".

Additional information

See: #3625 (comment)

Checklist

• The changes are tested and work correctly
• pre-commit checks pass / the code has been formatted using Black
• Tests have been added to verify that the new code works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.84%. Comparing base (ca569cf) to head (132db55).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #3630   +/-   ##
=======================================
  Coverage   89.84%   89.84%           
=======================================
  Files         322      322           
  Lines       10371    10371           
=======================================
  Hits         9318     9318           
  Misses       1053     1053           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.