config defaults: No longer apply ARTI_DEFAULTS in load()

This is redundant, because the defaults have to be supplied by the
config builders (usually via builder default attributes).

That this is actually done and correct is tested by the
`default_config()` test case in arti/src/cfg.rs.

crates/arti-config/src/lib.rs

@@ -169,10 +169,6 @@ impl ConfigurationSources {
     /// Load the configuration into a new [`config::Config`].
     pub fn load(&self) -> Result<config::Config, config::ConfigError> {
         let mut builder = config::Config::builder();
-        builder = builder.add_source(config::File::from_str(
-            options::ARTI_DEFAULTS,
-            config::FileFormat::Toml,
-        ));
         builder = add_sources(builder, &self.files, &self.options);
         builder.build()
     }

crates/arti/src/cfg.rs

@@ -197,7 +197,6 @@ mod test {
 
     #[test]
     fn default_config() {
-        // TODO: this is duplicate code.
         let cfg = config::Config::builder()
             .add_source(config::File::from_str(
                 ARTI_DEFAULTS,