DisplayTest: controller fine tuning. Disable u8g2 contrast management. (#772)

* DisplayTest: controller fine tuning. Disable u8g2 contrast management.
* DisplayTest: correct initial configuration values. Targets: cleanup dead code.
* DisplayTest: correct initial value for bias.

bootloader/targets/f7/target.c

@@ -191,7 +191,6 @@ void target_display_init() {
     u8g2_t fb;
     u8g2_Setup_st756x_erc(&fb, U8G2_R0, u8x8_hw_spi_stm32, u8g2_gpio_and_delay_stm32);
     u8g2_InitDisplay(&fb);
-    u8g2_SetContrast(&fb, 36);
     // Create payload
     u8g2_ClearBuffer(&fb);
     u8g2_SetDrawColor(&fb, 0x01);