Remove the alt_std_name option

This option introduced in #15820 allows a custom crate to be imported in
the place of std, but with the name std. I don't think there is any
value to this. At most it is confusing users of a driver that uses this option. There are no users of
this option on github. If anyone still needs it, they can emulate it
injecting #![no_core] in addition to their own prelude.

compiler/rustc_session/src/config.rs

@@ -770,7 +770,6 @@ impl Default for Options {
             externs: Externs(BTreeMap::new()),
             extern_dep_specs: ExternDepSpecs(BTreeMap::new()),
             crate_name: None,
-            alt_std_name: None,
             libs: Vec::new(),
             unstable_features: UnstableFeatures::Disallow,
             debug_assertions: true,
@@ -2382,7 +2381,6 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         unstable_features: UnstableFeatures::from_environment(crate_name.as_deref()),
         extern_dep_specs,
         crate_name,
-        alt_std_name: None,
         libs,
         debug_assertions,
         actually_rustdoc: false,