Remove flaky test

See https://github.com/rust-lang/rust/pull/81197 for what's going on
here; this is a temporary stopgap until someone has time to review the
proper fix.

src/test/rustdoc/issue-80893.rs

@@ -1,6 +0,0 @@
-// compile-flags: --test -Z unstable-options --test-builder true
-
-/// ```no_run
-/// This tests that `--test-builder` is accepted as a flag by rustdoc.
-/// ```
-pub struct Foo;