Delete gej_is_valid_var: unused outside tests

Summary:
This is a partial backport of secp256k1 [[bitcoin-core/secp256k1#808 | PR808]] : bitcoin-core/secp256k1@d7f39ae

Depends on D7656

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7657

src/secp256k1/src/group_impl.h

@@ -269,26 +269,6 @@ static int secp256k1_gej_is_infinity(const secp256k1_gej *a) {
     return a->infinity;
 }
 
-static int secp256k1_gej_is_valid_var(const secp256k1_gej *a) {
-    secp256k1_fe y2, x3, z2, z6;
-    if (a->infinity) {
-        return 0;
-    }
-    /** y^2 = x^3 + 7
-     *  (Y/Z^3)^2 = (X/Z^2)^3 + 7
-     *  Y^2 / Z^6 = X^3 / Z^6 + 7
-     *  Y^2 = X^3 + 7*Z^6
-     */
-    secp256k1_fe_sqr(&y2, &a->y);
-    secp256k1_fe_sqr(&x3, &a->x); secp256k1_fe_mul(&x3, &x3, &a->x);
-    secp256k1_fe_sqr(&z2, &a->z);
-    secp256k1_fe_sqr(&z6, &z2); secp256k1_fe_mul(&z6, &z6, &z2);
-    secp256k1_fe_mul_int(&z6, CURVE_B);
-    secp256k1_fe_add(&x3, &z6);
-    secp256k1_fe_normalize_weak(&x3);
-    return secp256k1_fe_equal_var(&y2, &x3);
-}
-
 static int secp256k1_ge_is_valid_var(const secp256k1_ge *a) {
     secp256k1_fe y2, x3, c;
     if (a->infinity) {

src/secp256k1/src/tests.c

@@ -2615,7 +2615,6 @@ void test_point_times_order(const secp256k1_gej *point) {
     secp256k1_ecmult(&ctx->ecmult_ctx, &res2, point, &nx, &nx); /* calc res2 = (order - x) * point + (order - x) * G; */
     secp256k1_gej_add_var(&res1, &res1, &res2, NULL);
     CHECK(secp256k1_gej_is_infinity(&res1));
-    CHECK(secp256k1_gej_is_valid_var(&res1) == 0);
     secp256k1_ge_set_gej(&res3, &res1);
     CHECK(secp256k1_ge_is_infinity(&res3));
     CHECK(secp256k1_ge_is_valid_var(&res3) == 0);
@@ -2647,7 +2646,6 @@ void run_point_times_order(void) {
             secp256k1_gej j;
             CHECK(secp256k1_ge_is_valid_var(&p));
             secp256k1_gej_set_ge(&j, &p);
-            CHECK(secp256k1_gej_is_valid_var(&j));
             test_point_times_order(&j);
         }
         secp256k1_fe_sqr(&x, &x);