configs: stm32mp1: cleanup config file

Remove the unnecessary comment after the CONFIG_SYS_BOOTM_LEN
migration to Kconfig.

Fixes: c45568cc ("Convert CONFIG_SYS_BOOTM_LEN to Kconfig")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>