[CBR-97] allow building 'post-mortem' tool from nix

[CodiePP]

Description

In benchmarking we depend on the tool 'post-mortem' for analysis of results.
Unfortunately, this executable cannot be built with nix, since we cannot pass the required flag to nix-build.
Therefore, we propose to remove this flag from tool's cabal file.

Linked issue

Type of change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🛠 New feature (non-breaking change which adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
• 🔨 New or improved tests for existing code
• ⛑ git-flow chore (backport, hotfix, etc)

Developer checklist

• [~] I have read the style guide document, and my code follows the code style of this project.
• [~] If my code deals with exceptions, it follows the guidelines.
• [~] I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on cardanodocs.com, amending the inline Haddock comments, any relevant README file or one of the document listed in the docs directory.
• [~] CHANGELOG entry has been added and is linked to the correct PR on GitHub.

Testing checklist

• [~] I have added tests to cover my changes.
• [~] All new and existing tests passed.

QA Steps

nix-build -A cardano-sl-tools -o post-mortem

[CodiePP]

post-mortem tool has been moved to its own space in PR #3670
this one can then be closed.