avfilter/af_arnndn: Remove superfluous qualifier

remove_doubling() returned a const float, but returning qualified types
is pointless anyway. This is all the more important given that  GCC 4.2.1
(and maybe other compilers, too) seems to treat this as an error as several
FATE units that don't compile any more show.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavfilter/af_arnndn.c

@@ -782,8 +782,8 @@ static float compute_pitch_gain(float xy, float xx, float yy)
 }
 
 static const int second_check[16] = {0, 0, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 5, 2, 3, 2};
-static const float remove_doubling(float *x, int maxperiod, int minperiod,
-                                   int N, int *T0_, int prev_period, float prev_gain)
+static float remove_doubling(float *x, int maxperiod, int minperiod, int N,
+                             int *T0_, int prev_period, float prev_gain)
 {
     int k, i, T, T0;
     float g, g0;