Remove proxy check in vec_is_list()

[DavisVaughan]

Part of #1208

The proxy check was originally suggested here #1003 (comment). I am not convinced that we need it, and it would help with list-rcrds outlined in #1208 to remove it.

I really like how simple vec_is_list() is now. It is very easy to explain.

Internally, we do not currently use vec_is_list() in any way that could cause issues after removing this restriction. We never proxy x after checking that it is a list with vec_is_list(). We only use it in:

• vec_unchop() to check that x is a list
• vec_unchop() to check that indices is a list
• list_sizes() to check that x is a list

I think all of these are valid and safe uses of vec_is_list()