libressl_2_7: remove, not maintained anymore

Stable LibreSSL releases are supported one year after their OpenBSD release.
OpenBSD 6.3 with this branch was released on 2018-04-01.

(cherry picked from commit cea163252adf3c893969e60d62659137ecd91e22)

pkgs/development/libraries/libressl/default.nix

···
 
 in {
 
-  libressl_2_7 = generic {
-    version = "2.7.5";
-    sha256 = "0h60bcx7k72171dwpx4vsbsrxxz9c18v75lh5fj600gghn6h7rdy";
-  };
-
   libressl_2_8 = generic {
     version = "2.8.3";
     sha256 = "0xw4z4z6m7lyf1r4m2w2w1k7as791c04ygnfk4d7d0ki0h9hnr4v";

pkgs/top-level/all-packages.nix

···
   openvdb = callPackage ../development/libraries/openvdb {};
 
   inherit (callPackages ../development/libraries/libressl { })
-    libressl_2_7
     libressl_2_8
     libressl_2_9;