and columns"

This reverts commit 073c4e1cc712064e626914fa4a5a8061f903a637.

https://bugs.ruby-lang.org/issues/6012#note-31
> we will cancel this feature in 4.0 because of design ambiguities
> such as whether to return column positions in bytes or characters as
> in [#21783].

[#21783]: https://bugs.ruby-lang.org/issues/21783

Previously, ruby2_keywords could be used on a method or proc with
post arguments, but I don't think the behavior is desired:

```ruby
def a(*c, **kw) [c, kw] end
def b(*a, b) a(*a, b) end
ruby2_keywords(:b)

b({foo: 1}, bar: 1)
```

This changes ruby2_keywords to emit a warning and not set the
flag on a method/proc with post arguments.

While here, fix the ruby2_keywords specs for warnings, since they
weren't testing what they should be testing.  They all warned
because the method didn't accept a rest argument, not because it
accepted a keyword or keyword rest argument.