Remove unnecessary use of include! in rustdoc test

Using `include!` shouldn't affect the test. It was only added because:

> I replicated how it was performed in libstd. Since it's the main
> target of this fix, I thought it was the best way.

<https://github.com/rust-lang/rust/pull/52827/files#r207647331>

But it's unnecessary and adds unnecessary indirection.

src/test/rustdoc/primitive-generic-impl.rs

@@ -1,7 +0,0 @@
-#![feature(rustdoc_internals)]
-
-#![crate_name = "foo"]
-
-include!("primitive/primitive-generic-impl.rs");
-
-// @has foo/primitive.i32.html '//div[@id="impl-ToString"]//h3[@class="code-header in-band"]' 'impl<T> ToString for T'

src/test/rustdoc/primitive/primitive-generic-impl.rs

@@ -1,6 +1,8 @@
 #![feature(rustdoc_internals)]
 
-// FIXME: this test doesn't test anything
+#![crate_name = "foo"]
+
+// @has foo/primitive.i32.html '//div[@id="impl-ToString"]//h3[@class="code-header in-band"]' 'impl<T> ToString for T'
 
 #[doc(primitive = "i32")]
 /// Some useless docs, wouhou!