refactor(bl)!: remove unused plat_try_next_boot_source

The plat_try_next_boot_source() API is not used by any upstream platform
and not used by platforms that asked for this API. It is then removed.
It will be replaced with a more generic interface in next patch.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I298c7acace8c5efb3c66422d8d9280ecd08e5ade
4 files changed