This reverts commit 5ff9766908.
The esp-onen-sdk adds the fix for force-requiring user_rf_pre_init() [1].
So we need to drop the user_rf_pre_init function in user_main.c to prevent
multiple definition of `user_rf_pre_init' error.
[1] 4e4590de89
This patch fixes below build error with SDK-1.1.0:
esp-open-sdk/sdk/lib/libmain.a(app_main.o): In function `user_uart_wait_tx_fifo_empty':
(.irom0.text+0x34c): undefined reference to `user_rf_pre_init'
According to esp_iot_sdk_v1.1.0_15_05_26 Release Note:
user_rf_pre_init has to be added in user_main.c, user can call
system_phy_set_rfoption to set configuration of RF in it.