Remove `inf` local variable.

Apparently `inf` is a macro on iOS for `std::numeric_limits<T>::infinity()`,
causing a compile error here. We don't need the local anyways since it's
only used in one spot.
diff --git a/Eigen/src/Core/MathFunctionsImpl.h b/Eigen/src/Core/MathFunctionsImpl.h
index 8ecddeb..9222285 100644
--- a/Eigen/src/Core/MathFunctionsImpl.h
+++ b/Eigen/src/Core/MathFunctionsImpl.h
@@ -132,8 +132,7 @@
 
   // Special case of isinf(y)
   if ((numext::isinf)(y)) {
-    const T inf = std::numeric_limits<T>::infinity();
-    return std::complex<T>(inf, y);
+    return std::complex<T>(std::numeric_limits<T>::infinity(), y);
   }
 
   T w = numext::sqrt(cst_half * (numext::abs(x) + numext::abs(z)));