Remove "out" from the listArg-list (#1338)

"out" as an attribute seems to be a list and a string literal, varying between different rules.

From a lexical standpoint, it would be more likely to be a singluar value as the the wording is singular, and it would be more expected for a rule which provides multiple outs to use the "outs" attribute.

Metrics (based on Google internal use) show use of the attribute are ~5% lists, and ~95% string.