Drop no longer used variables from the build system #1196

1. 
   hebasto commented at 9:43 am on January 19, 2023: member

   SECP_INCLUDES, SECP_LIBS, SECP_TEST_LIBS and SECP_TEST_INCLUDES were introduced in 78cd96b15153e209cf4829a511f9efdfdcf7e4d0.

   The last usage of the SECP_TEST_{LIBS,INCLUDE} variables was removed in #983.

   The last usage of the SECP_LIBS variable was removed in #831.

   The last usage of the SECP_INCLUDE variable was removed in #1169.

2. Drop no longer used `SECP_TEST_{LIBS,INCLUDE}` variables

   The last usage of the `SECP_TEST_{LIBS,INCLUDE}` variables was removed
   in https://github.com/bitcoin-core/secp256k1/pull/983.

   613626f94c
3. Drop no longer used `SECP_{LIBS,INCLUDE}` variables

   The last usage of the `SECP_INCLUDE` variable was removed
   in https://github.com/bitcoin-core/secp256k1/pull/1169.

   2cd4e3c0a9
4. real-or-random approved
5. 
   real-or-random commented at 10:58 am on January 19, 2023: contributor

   utACK 2cd4e3c0a976c78c1619fc7456fcc4eaa92897a9
6. 
   sipa commented at 3:56 pm on January 19, 2023: contributor

   utACK 2cd4e3c0a976c78c1619fc7456fcc4eaa92897a9
7. sipa merged this on Jan 19, 2023
8. sipa closed this on Jan 19, 2023

   ---

9. hebasto deleted the branch on Jan 19, 2023
10. dhruv referenced this in commit 4d33046ce3 on Feb 1, 2023
11. dhruv referenced this in commit 55e7f2cf2b on Feb 2, 2023
12. stratospher referenced this in commit 647f63669e on Feb 6, 2023
13. dhruv referenced this in commit a4351c0df6 on Feb 20, 2023
14. stratospher referenced this in commit 23f825fc8b on Feb 21, 2023
15. hebasto referenced this in commit 7c0cc5d976 on Mar 7, 2023
16. dhruv referenced this in commit a5df79db12 on Mar 7, 2023
17. dhruv referenced this in commit 77b510d84c on Mar 7, 2023
18. sipa referenced this in commit 763079a3f1 on Mar 8, 2023
19. div72 referenced this in commit 945b094575 on Mar 14, 2023
20. vmta referenced this in commit e1120c94a1 on Jun 4, 2023
21. vmta referenced this in commit 8f03457eed on Jul 1, 2023
22. jonasnick cross-referenced this on Jul 20, 2023 from issue Upstream PRs 1160, 1193, 1169, 1190, 1192, 1194, 1196, 1195, 1170, 1172, 1200, 1199, 1203, 1201, 1206, 1078, 1209, 979, 1212, 1218, 1217, 1221, 1222 by jonasnick

Contributors
hebasto real-or-random sipa