Remove marker testing library

`rust-marker` hasn't been updated in a while, which makes maintaining
the `marker` libary a little bit of a burden.

examples/README.md

@@ -63,7 +63,6 @@ The example libraries are separated into the following three categories:
 | Example                            | Description/check                                      |
 | ---------------------------------- | ------------------------------------------------------ |
 | [`clippy`](./testing/clippy)       | All of the Clippy lints as a Dylint library            |
-| [`marker`](./testing/marker)       | Marker lints run from a Dylint library                 |
 | [`straggler`](./testing/straggler) | A lint that uses an old toolchain for testing purposes |
 
 **Notes**

examples/src/lib.rs

@@ -12,12 +12,6 @@ mod test {
     fn examples() {
         for path in iter(false).unwrap() {
             let path = path.unwrap();
-            // smoelius: Skip the `marker` test for now. `marker` uses nightly-2023-11-16, which is
-            // Rust 1.76. But `rustfix` 0.8.5 (used by `dylint_testing`) requires rustc 1.77 or
-            // newer.
-            if path.ends_with("testing/marker") {
-                continue;
-            }
             let file_name = path.file_name().unwrap();
             // smoelius: Pass `--lib --tests` to `cargo test` to avoid the potential filename
             // collision associated with building the examples.
@@ -91,9 +85,6 @@ mod test {
         let mut prev = None;
         for path in iter(true).unwrap() {
             let path = path.unwrap();
-            if path.file_name() == Some(OsStr::new("marker")) {
-                continue;
-            }
             if path.file_name() == Some(OsStr::new("straggler")) {
                 continue;
             }