Delete to_string_in_format_args_incremental.rs

It fails CI and passes locally. It passes random directores in
`-Cincremental` so maybe something's up. It shouldn't block us here.

tests/ui/to_string_in_format_args_incremental.rs

@@ -1,9 +0,0 @@
-//@run-rustfix
-//@compile-flags: -C incremental=target/debug/test/incr
-
-// see https://github.com/rust-lang/rust-clippy/issues/10969
-
-fn main() {
-    let s = "Hello, world!";
-    println!("{}", s.to_string());
-}

tests/ui/to_string_in_format_args_incremental.stderr

@@ -1,10 +0,0 @@
-error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/to_string_in_format_args_incremental.rs:8:21
-   |
-LL |     println!("{}", s.to_string());
-   |                     ^^^^^^^^^^^^ help: remove this
-   |
-   = note: `-D clippy::to-string-in-format-args` implied by `-D warnings`
-
-error: aborting due to previous error
-