Remove secp256k1_fe_const_b

[roconnor-blockstream]

It's last use was eliminated in #1217.

[real-or-random]

utACK d6be470

[sipa]

src/tests.c: In function 'ecmult_const_mult_xonly':
src/tests.c:4501:35: error: 'secp256k1_fe_const_b' undeclared (first use in this function); did you mean 'secp256k1_ge_const_g'?
 4501 |             secp256k1_fe_add(&c, &secp256k1_fe_const_b);
      |                                   ^~~~~~~~~~~~~~~~~~~~
      |                                   secp256k1_ge_const_g

It would appear that #1118 added another use back (which can easily be converted to secp256k1_fe_add_int() instead).

[real-or-random]

src/tests.c: In function 'ecmult_const_mult_xonly':
src/tests.c:4501:35: error: 'secp256k1_fe_const_b' undeclared (first use in this function); did you mean 'secp256k1_ge_const_g'?
 4501 |             secp256k1_fe_add(&c, &secp256k1_fe_const_b);
      |                                   ^~~~~~~~~~~~~~~~~~~~
      |                                   secp256k1_ge_const_g

It would appear that #1118 added another use back (which can easily be converted to secp256k1_fe_add_int() instead.)

By the way, this shows that it's a good idea to remove secp256k1_fe_const_b to make sure that noone will use it in the future.

[sipa]

See #1283 for a fix.