Remove LintDiagnostic::msg

* instead simply set the primary message inside the lint decorator functions
* it used to be this way before [#]101986 which introduced `msg` to prevent
  good path delayed bugs (which no longer exist) from firing under certain
  circumstances when lints were suppressed / silenced
* this is no longer necessary for various reasons I presume
* it shaves off complexity and makes further changes easier to implement

compiler/rustc_pattern_analysis/src/lints.rs

@@ -99,7 +99,6 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'p, 'tcx>(
 
                 use rustc_errors::LintDiagnostic;
                 let mut err = rcx.tcx.dcx().struct_span_warn(arm.pat.data().span, "");
-                err.primary_message(decorator.msg());
                 decorator.decorate_lint(&mut err);
                 err.emit();
             }