wifi: esp32: remove unused ESP32_WIFI_{SSID,PSK}

Those Kconfig options are no longer used after ESP32 WiFi driver was
converted to Zephyr WiFi mgmt and the removal of platform specific
`samples/boards/esp32/wifi`.

Fixes: d524015f ("samples: boards: remove esp32 wifi sample code")

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>