Remove WM_NO_TEMP compilation warning

[PBrunot]

WiFiManager always generates a warning on ESP32 due to missing temperature sensor.

With platformio it is not possible to configure the -Wno-cpp flag per library. I want to compile my project with -Wall -Wextra , and having a warning I cannot solve bugs me. I didn't see the point of the warning of the user of the library anyways.

This PR remove the #warning instruction.