configs: stm32f746-disco: remove a useless comment

Commit 8fc78fc7 ("configs: migrate CONFIG_BMP_16/24/32BPP to defconfigs")
made the comment useless.

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>