Remove and tidy lint against rust str escape characters in ftl files

[est31]

From #109677:

using \n in the messages.ftl file doesn't work

Grepping turns up multiple \n characters in the ftl files. These don't work in fluent however, instead you are supposed to do newlines directly. There should be a tidy lint that forbids such characters, as it's a easy mistake to do during porting. I also see \" escapes that seem not legit.

@rustbot label A-translation

cc @davidtwco