configs: stm32f746-disco: Remove CONFIG_SYS_UBOOT_START flag

configs: stm32f746-disco: Remove CONFIG_SYS_UBOOT_START flag

By pressing "c" key during SPL execution, we force U-boot execution
instead of a kernel XIP image.

This fixes a hard fault when booting stm32f746-disco in SPL with "c"
key pressed during SPL execution.

U-Boot SPL 2022.10-rc5-00009-g40d02baa91 (Sep 20 2022 - 17:21:21 +0200)
Trying to boot from XIP
Hard fault
pc : 080083fc    lr : 08000d1b    xPSR : 21000000
r12 : 2004f108   r3 : 080083fd    r2 : 00000028
r1 : 2004f0c8    r0 : 2004f0e4
Resetting CPU ...

This is due to SYS_UBOOT_START flag set to 0x080083FD which is not correct.
If unset, SYS_UBOOT_START is set by default to CONFIG_SYS_TEXT_BASE
which match with our requirement.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>