group: remove unneeded normalize_weak in secp256k1_gej_eq_x_var

By requiring that the input group element's X coordinate (`a->x`) has a
magnitude of <= 31, the normalize_weak call and also the field element
variable `r2` are not needed anymore and hence can be dropped.

This makes ECDSA verification a little faster. On my machine:

Benchmark                     ,    Min(us)    ,    Avg(us)    ,    Max(us)

[ master ]
ecdsa_verify                  ,    68.9       ,    72.4       ,    77.8

[ PR ]
ecdsa_verify                  ,    65.4       ,    68.2       ,    76.6

src/group.h

@@ -100,7 +100,8 @@ static void secp256k1_gej_set_ge(secp256k1_gej *r, const secp256k1_ge *a);
 /** Check two group elements (jacobian) for equality in variable time. */
 static int secp256k1_gej_eq_var(const secp256k1_gej *a, const secp256k1_gej *b);
 
-/** Compare the X coordinate of a group element (jacobian). */
+/** Compare the X coordinate of a group element (jacobian).
+  * The mangitude of the group element's X coordinate must not exceed 31. */
 static int secp256k1_gej_eq_x_var(const secp256k1_fe *x, const secp256k1_gej *a);
 
 /** Set r equal to the inverse of a (i.e., mirrored around the X axis) */

src/group_impl.h

@@ -314,13 +314,17 @@ static int secp256k1_gej_eq_var(const secp256k1_gej *a, const secp256k1_gej *b)
 }
 
 static int secp256k1_gej_eq_x_var(const secp256k1_fe *x, const secp256k1_gej *a) {
-    secp256k1_fe r, r2;
+    secp256k1_fe r;
+
+#ifdef VERIFY
     secp256k1_fe_verify(x);
+    VERIFY_CHECK(a->x.magnitude <= 31);
     secp256k1_gej_verify(a);
     VERIFY_CHECK(!a->infinity);
+#endif
+
     secp256k1_fe_sqr(&r, &a->z); secp256k1_fe_mul(&r, &r, x);
-    r2 = a->x; secp256k1_fe_normalize_weak(&r2);
-    return secp256k1_fe_equal_var(&r, &r2);
+    return secp256k1_fe_equal_var(&r, &a->x);
 }
 
 static void secp256k1_gej_neg(secp256k1_gej *r, const secp256k1_gej *a) {