build: drop explicit awkward dependency

[alexander-held]

Following #408 there was only one place left where awkward was used in the cabinetry code base, in cabinetry.tabulate. The use of awkward there was not crucial, so this is replaced by pure numpy here. This means that the explicit awkward dependency can be dropped from cabinetry. It still enters in cabinetry[contrib] via uproot.

* drop explicit dependency on awkward
* remove awkward HEAD tests from nightly CI tests

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (7b9b6af) 100.00% compared to head (d5056f0) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #419 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 2072 2072 Branches 337 338 +1 ========================================= Hits 2072 2072 ``` | [Impacted Files](https://app.codecov.io/gh/scikit-hep/cabinetry/pull/419?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | Coverage Δ | | |---|---|---| | [src/cabinetry/tabulate.py](https://app.codecov.io/gh/scikit-hep/cabinetry/pull/419?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep#diff-c3JjL2NhYmluZXRyeS90YWJ1bGF0ZS5weQ==) | `100.00% <100.00%> (ø)` | |

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