cli: remove unreachable yaml.safe_loads path

[Elias0419]

Hi

_generate_yaml() in sceptre/cli/helpers.py calls a nonexistent API, yaml.safe_loads (and has for seven years)

https://github.com/Sceptre/sceptre/blame/master/sceptre/cli/helpers.py#L157

Since that code path is unused, it should be removed:

diff --git a/sceptre/cli/helpers.py b/sceptre/cli/helpers.py
index 4fd98c0..168cf3e 100644
--- a/sceptre/cli/helpers.py
+++ b/sceptre/cli/helpers.py
@@ -153,10 +153,7 @@ def _generate_yaml(stream):
         return yaml.dump(stream, **kwargs)
 
     else:
-        try:
-            return yaml.safe_loads(stream)
-        except Exception:
-            return stream
+        return stream