spl: remove duplicate SPL_MD5 option

There is another SPL_MD5 option defined in lib/Kconfig.
Renaming SPL_MD5_SUPPORT introduced duplicate option with
different description. As for now FIT and hash algorithm options
are not related to each others, removing a duplicate option seems OK.

Fixes: 4b00fd1a ("Kconfig: Rename SPL_MD5_SUPPORT to SPL_MD5")
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>