xtensa-build-zephyr.py: don't pass -c $plat.toml to rimage

Let west/sign.py find the appropriate .toml file.

This prepares for .toml modularization thesofproject#8490

Signed-off-by: Marc Herbert <marc.herbert@intel.com>
(cherry picked from commit d661aad)

scripts/xtensa-build-zephyr.py

@@ -625,10 +625,6 @@ def rimage_options(platform_dict):
 	#                         test_00_01_load_fw_and_check_version
 	opts.append(("-b", "1"))
 
-	rimage_desc = platform_dict["name"] + ".toml"
-
-	opts.append(("-c", str(RIMAGE_SOURCE_DIR / "config" / rimage_desc)))
-
 	return opts