Influx-related static methods

[Salvialf]

Some (most of) Influx-related methods in the cmd class should be made static for consistency — for example dropDatabaseInflux or historyInfluxAll.

But do we even need to keep this functionality at all? It probably belongs in an external plugin; there’s no real reason to maintain this code directly in the Jeedom core.

Edit : Related to #3039

[Salvialf]

dropInfluxDatabase will be corrected by #3259 and historyInfluxAll by #3260, but the question of whether this should remain in the core is still open.