exp diff: Remove --old option.

[daavoo]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

For consistency (#5515) with output format of other diff_table uses.

Docs P.R: iterative/dvc.org#2988

---

Before:

$ dvc exp diff exp-1dad0 exp-1df77

Path         Metric    Value    Change
scores.json  auc       0.51676  -0.060799

Path         Param                   Value    Change
params.yaml  featurize.max_features  500      -1500

After:

$ dvc exp diff exp-1dad0 exp-1df77

Path         Metric  exp-1dad0  exp-1df77  Change
scores.json  auc     0.577559   0.51676    -0.060799

Path         Param                   exp-1dad0  exp-1df77  Change
params.yaml  featurize.max_features  2000       500        -1500