cpu/esp32: switch from O2 to Os

In RIOT-OS#12955 optimization was switched to O2 because with the '-Os'
option, the ESP32 hangs sporadically in 'tests/bench*' if
interrupts where disabled too early by benchmark tests.

Since it hasn't been reproduced since and in RIOT-OS#13196 O2 was causing
un-explained hardfaults, since the aforementioned issue could not
be reproduced we switch back to Os by removing O2, as Os will be
used by default.

cpu/esp32/Makefile.include

@@ -7,10 +7,6 @@ endif
 
 ESP_SDK_DIR = $(ESP32_SDK_DIR)
 
-# With the '-Os' option, the ESP32 hangs sporadically in 'tests/bench*' if
-# interrupts are disabled too early by benchmark tests.
-CFLAGS_OPT ?= -O2
-
 # ESP32 specific flashing options
 FLASH_CHIP  = esp32
 FLASH_MODE ?= dout