In #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 #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.
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