Remove special casing for some meta attributes

Descriptions and licenses are handled by Cargo now, so there's no reason
to keep these attributes around.
Update docstrings for UnicodeChar::is_digit => is_numeric rename
auto merge of #19343 : sfackler/rust/less-special-attrs, r=alexcrichton

Descriptions and licenses are handled by Cargo now, so there's no reason
to keep these attributes around.
auto merge of #19348 : SimonSapin/rust/patch-9, r=huonw