binman: Drop algo check in CheckSetHashValue()

The CheckAddHashValue() function is always called before this one, so
the algorithm check is never used. Replace it with an assert to avoid a
coverage error.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/state.py b/tools/binman/state.py
index 6772d36..f4d885c 100644
--- a/tools/binman/state.py
+++ b/tools/binman/state.py
@@ -411,8 +411,7 @@
             m = hashlib.sha256()
             m.update(get_data_func())
             data = m.digest()
-        if data is None:
-            raise ValueError(f"Node '{node.path}': Unknown hash algorithm '{algo}'")
+        assert data
         for n in GetUpdateNodes(hash_node):
             n.SetData('value', data)