boards: arm: stm32f3 boards have no FPU by default

Remove the CONFIG_FPU for the stm32f3_disco and
stm32373c eval boards.
The FPU exists like on many other stm32 series
but CONFIG_FPU has not to be defined here.

Signed-off-by: Francois Ramu <francois.ramu@st.com>