Remove proc-macro2-diagnostics dependency

[dtolnay]

The code using proc-macro2-diagnostics produces worse/spurious diagnostics on both stable and nightly compilers. Example:

use test_case::test_case;

#[test_case(1 => is not not eq 1 ; "repro")]
fn id(input: i32) -> i32 {
    input
}

Before:

$ cargo +nightly test

error: multiple negations on single item are forbidden
 --> src/main.rs:3:1
  |
3 | #[test_case(1 => is not not eq 1 ; "repro")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the attribute macro `test_case` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0308]: mismatched types
 --> src/main.rs:3:45
  |
3 | #[test_case(1 => is not not eq 1 ; "repro")]
  | --------------------------------------------^ expected `bool`, found `()`
  | |
  | implicitly returns `()` as its body has no tail or `return` expression
  |
  = note: this error originates in the attribute macro `test_case` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0308`.

$ cargo +stable test

error: multiple negations on single item are forbidden
 --> src/main.rs:3:1
  |
3 | #[test_case(1 => is not not eq 1 ; "repro")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the attribute macro `test_case` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0308]: mismatched types
 --> src/main.rs:3:1
  |
3 | #[test_case(1 => is not not eq 1 ; "repro")]
  | ^-------------------------------------------
  | |
  | expected `bool`, found `()`
  | implicitly returns `()` as its body has no tail or `return` expression
  |
  = note: this error originates in the attribute macro `test_case` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0308`.

After:

$ cargo test  # (same on both)

error: multiple negations on single item are forbidden
 --> src/main.rs:3:1
  |
3 | #[test_case(1 => is not not eq 1 ; "repro")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the attribute macro `test_case` (in Nightly builds, run with -Z macro-backtrace for more info)

[luke-biel]

An oversight on our side. Tested this locally and it checks out. Thanks for contrib.