ARM: dts: stm32f769-disco: remove the dsi_host node

The node has become useless, as described in the
commit 754815b8 ("video: stm32: remove the compatible "synopsys, dw-mipi-dsi" support")

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>