refactor: remove visualization and fanova feature flags

Always build visualization and fanova code unconditionally.

tests/fanova_tests.rs

@@ -1,7 +1,5 @@
 //! Integration tests for fANOVA parameter importance.
 
-#![cfg(feature = "fanova")]
-
 use optimizer::prelude::*;
 
 #[test]

tests/visualization_tests.rs

@@ -1,5 +1,3 @@
-#![cfg(feature = "visualization")]
-
 use optimizer::parameter::{FloatParam, IntParam, Parameter};
 use optimizer::sampler::random::RandomSampler;
 use optimizer::{Direction, Study, generate_html_report};