dts: arm: st: f4: Remove redundant ram-size for stm32f469

This parameter is defined for stm32f405 which and is included into
stm32f469. The include order is as follows:

st/f4/stm32f4.dtsi
st/f4/stm32f401.dtsi
st/f4/stm32f405.dtsi <-- ram_size defined here
st/f4/stm32f407.dtsi
st/f4/stm32f427.dtsi
st/f4/stm32f429.dtsi
st/f4/stm32f469.dtsi

Refer to Application Note AN4879 from ST to view ram size for each MCU.

Signed-off-by: Guðni Már Gilbert <gudni.m.g@gmail.com>