board: stm32: Remove the bi_boot_params initialization

The stm32 platforms never had to support an ATAGs-based Linux Kernel,
so remove the bi_boot_params initialization.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>