librustc: Remove the `&LIFETIME EXPR` production from the language.

This was parsed by the parser but completely ignored; not even stored in
the AST!

This breaks code that looks like:

    static X: &'static [u8] = &'static [1, 2, 3];

Change this code to the shorter:

    static X: &'static [u8] = &[1, 2, 3];

Closes #15312.

[breaking-change]

r? @nick29581