chore: remove MULTI_CONSOLE_API references

MULTI_CONSOLE_API have been removed long time ago by commit 5b6ebeec
("Remove MULTI_CONSOLE_API flag and references to it") that's why remove
references in platform.mk files and also in one rst which is not valid
anymore.

Change-Id: I45f8e7db0a14ce63de62509100d8159b7aca2657
Signed-off-by: Michal Simek <michal.simek@amd.com>