openvpn: cleanup unnecessary generic function

[nevivurn]

Description of changes

• The generic function was added in openvpn: 2.4.9 -> 2.5.0 #103019 to support packaging both 2.5.x and 2.4.x in nixpkgs, but openvpn_24 has since been removed in openvpn_24: remove #212184.
• While I was at it, removed ${pname} (Tracking issue: superfluous use of pname #277994)
• Rebuilds are due to reordering of openssl in buildInputs

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[nevivurn]

Sorry about the noise, I have addressed the review comments.

[infinisil]

You can ignore the pkgs/by-name failure, the PR can be merged even if it's red without causing problems. The fix for this is already merged with #283017, though it'll take some time for it to be applied.

[infinisil]

Btw this kind of pkgs/by-name check failure is fixed for future PRs now that #284765 is merged :)