remove support for the #[start] attribute

compiler/rustc_codegen_gcc/src/context.rs

@@ -513,7 +513,6 @@ impl<'gcc, 'tcx> MiscCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         } else {
             // If the symbol already exists, it is an error: for example, the user wrote
             // #[no_mangle] extern "C" fn main(..) {..}
-            // instead of #[start]
             None
         }
     }