tests/rustdoc-json: Remove some needless uses of #![no_core].

Done by removing all uses of `#![no_core]`, and the reverting the ones
that failed. More involved ones are in a later commit.

tests/rustdoc-json/stripped_modules.rs

@@ -1,6 +1,3 @@
-#![no_core]
-#![feature(no_core)]
-
 // @!has "$.index[*][?(@.name=='no_pub_inner')]"
 mod no_pub_inner {
     fn priv_inner() {}