prog_loaders: Remove CONFIG_MIRROR_PAYLOAD_TO_RAM_BEFORE_LOADING

This option is not used on any platform and is not user-visible. It
seems that it has not been used by anyone for a long time (maybe ever).
Let's get rid of it to make future CBFS / program loader development
simpler.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I2fa4d6d6f7c1d7a5ba552177b45e890b70008f36
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39442
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/include/program_loading.h

@@ -205,9 +205,6 @@ void payload_load(void);
 /* Run the loaded payload. */
 void payload_run(void);
 
-/* Mirror the payload to be loaded. */
-void mirror_payload(struct prog *payload);
-
 /*
  * selfload() and selfload_check() load payloads into memory.
  * selfload() does not check the payload to see if it targets memory.