Remove redundant redeclarations causing -Wredundant-decls warnings

Fixes #1469

PiperOrigin-RevId: 538534014
Change-Id: Iab84f73c0a0665355b614e85c62f686dc54c8671

absl/numeric/int128_have_intrinsic.inc

@@ -162,9 +162,6 @@ inline int128::operator long double() const {
 }
 }
 
 
 #else  // Clang on PowerPC
 #else  // Clang on PowerPC
-// Forward declaration for conversion operators to floating point types.
-constexpr int128 operator-(int128 v);
-constexpr bool operator!=(int128 lhs, int128 rhs);
 
 
 inline int128::operator float() const {
 inline int128::operator float() const {
   // We must convert the absolute value and then negate as needed, because
   // We must convert the absolute value and then negate as needed, because