[chore](trx-backend-soapysdr): remove unused smoothstep01

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs

@@ -136,11 +136,6 @@ fn dot_product(a: &[f32], b: &[f32]) -> f32 {
     dot_product_scalar(a, b)
 }
 
-fn smoothstep01(x: f32) -> f32 {
-    let x = x.clamp(0.0, 1.0);
-    x * x * (3.0 - 2.0 * x)
-}
-
 #[inline]
 fn fast_atan2(y: f32, x: f32) -> f32 {
     if x == 0.0 {