Mustool MT15/29: don't tag air sensors as diagnostic

Since the device's primary purpose is to measure air quality, anything
related to that should not be diagnostic.

Comment on #3054

custom_components/tuya_local/devices/mustool_mt15mt29_airbox.yaml

@@ -111,7 +111,6 @@ entities:
             value: mute
   - entity: sensor
     class: volatile_organic_compounds
-    category: diagnostic
     dps:
       - id: 101
         type: integer
@@ -121,7 +120,6 @@ entities:
         class: measurement
   - entity: sensor
     class: carbon_monoxide
-    category: diagnostic
     dps:
       - id: 102
         type: integer
@@ -187,7 +185,6 @@ entities:
         name: switch
   - entity: sensor
     name: PM 0.3
-    category: diagnostic
     dps:
       - id: 107
         type: integer