Remove incorect use of rstrip (#1262)

* Remove incorect use of rstrip

* Add back check to not change labels of targets not ending with _proto

Author: liucijus

scala_proto/private/scala_proto_aspect.bzl

@@ -160,12 +160,18 @@ def _phase_aspect_provider(ctx, p):
         },
     )
 
+def _strip_suffix(str, suffix):
+    if str.endswith(suffix):
+        return str[:-len(suffix)]
+    else:
+        return str
+
 def _phase_stamp_label(ctx, p):
     rule_label = str(p.target.label)
     toolchain = ctx.toolchains["@io_bazel_rules_scala//scala_proto:toolchain_type"]
 
     if toolchain.stamp_by_convention and rule_label.endswith("_proto"):
-        return rule_label.rstrip("_proto") + "_scala_proto"
+        return _strip_suffix(rule_label, "_proto") + "_scala_proto"
     else:
         return rule_label