HAL_F4Light: removed SPL submodule

[tridge]

this submodule contains files licensed under the ST liberty_v2 license,
which is not compatible with the GPL

Unfortunately removing this breaks the F4Light build. People wanting to
build F4Light will need modifications locally

[auturgy]

I completely agree with the rationale behind this. It will need a dapo post to explain though, I think.

[tridge]

unfortunately even after we apply this PR we will still have quite a few files licensed under the ST liberty_v2 license. For example:
https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_F4Light/hardware/STM32_USB_Driver/usb_core.c#L17
I think we will have to go further and remove F4Light HAL. If the HAL got cleaned up to remove all ST code that is under this license then it could come back in.

[tridge]

This PR has now been expanded to remove the two submodules and the ST code for USB. It doesn't remove the rest of F4Light.
I'm currently thinking we can leave the rest in to make it easier for @night-ghost to maintain F4Light out of our tree if he wants to.
That also will make it easier to re-enable F4Light if either ST changes their license or @night-ghost rewrites code to not use the ST licensed code.
Also note that we have written to ST to ask them about the license.