AP_Math: move is_valid_octal into adsb

This doesn't ensure the value is octal digits - there's more magic in
it.

libraries/AP_Math/AP_Math.cpp

@@ -321,23 +321,6 @@ Vector3f rand_vec3f(void)
 }
 #endif
 
-bool is_valid_octal(uint16_t octal)
-{
-    // treat "octal" as decimal and test if any decimal digit is > 7
-    if (octal > 7777) {
-        return false;
-    } else if (octal % 10 > 7) {
-        return false;
-    } else if ((octal % 100)/10 > 7) {
-        return false;
-    } else if ((octal % 1000)/100 > 7) {
-        return false;
-    } else if ((octal % 10000)/1000 > 7) {
-        return false;
-    }
-    return true;
-}
-
 /*
   return true if two rotations are equivalent
   This copes with the fact that we have some duplicates, like ROLL_180_YAW_90 and PITCH_180_YAW_270

libraries/AP_Math/AP_Math.h

@@ -274,9 +274,6 @@ float rand_float(void);
 // generate a random Vector3f of size 1
 Vector3f rand_vec3f(void);
 
-// confirm a value is a valid octal value
-bool is_valid_octal(uint16_t octal) WARN_IF_UNUSED;
-
 // return true if two rotations are equal
 bool rotation_equal(enum Rotation r1, enum Rotation r2) WARN_IF_UNUSED;