[RTL_433 Discovery] Do not store the devices in memory if discovery off

[1technophile]

Description:

Avoid storing devices in the vector if the discovery is off, the goal is to avoid a growing vector memory when the user stops the discovery

Checklist:

• The pull request is done against the latest development branch
• Only one feature/fix was added per PR and the code change compiles without warnings
• I accept the DCO.