test: Remove compile-fail/export-boot.rs

This was a rustboot-specific test which is covered by other tests in rustc.

src/test/compile-fail/export-boot.rs

@@ -1,21 +0,0 @@
-// xfail-stage0
-// xfail-stage1
-// xfail-stage2
-// error-pattern: unknown module item
-
-// rustboot has a different error message than rustc
-// this test can die with rustboot, or rustc's error can change
-
-mod foo {
-  export x;
-  fn x(int y) {
-    log y;
-  }
-  fn z(int y) {
-    log y;
-  }
-}
-
-fn main() {
-  foo::z(10);
-}