Remove Nexus publishing plugin from buildSrc

[0xnm]

What does this PR do?

Tiny change: Nexus publishing plugin actually shouldn't be in the buildSrc classpath and so we can move it out and use it via plugin reference.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• [ ] Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.08%. Comparing base (e3a82d9) to head (d00d23e). Report is 6 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #2122 +/- ## =========================================== - Coverage 70.23% 70.08% -0.14% =========================================== Files 721 721 Lines 26769 26737 -32 Branches 4487 4482 -5 =========================================== - Hits 18799 18738 -61 - Misses 6719 6749 +30 + Partials 1251 1250 -1 ``` [see 31 files with indirect coverage changes](https://app.codecov.io/gh/DataDog/dd-sdk-android/pull/2122/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog)