XAS: Migrate XAS App to New Repository

[PNOGillespie]

Removes the XAS app from the AiiDAlab-QE repository as part of migrating the app to aiidaplugins/aiida-qe-xspec.

Additionally updates some minor details of the aiida-qe-xspec entry of plugins.yaml.

[PNOGillespie]

@superstar54 I updated the description for the XSpec plugin (both the title and GitHub repo) since I noticed the details were not quite correct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.68%. Comparing base (7bea74e) to head (9352e3b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1139      +/-   ##
==========================================
+ Coverage   71.94%   73.68%   +1.73%     
==========================================
  Files         105       94      -11     
  Lines        6886     6433     -453     
==========================================
- Hits         4954     4740     -214     
+ Misses       1932     1693     -239     

Flag	Coverage Δ
python-3.11	73.68% <ø> (+1.73%)	⬆️
python-3.9	73.68% <ø> (+1.75%)	⬆️

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[superstar54]

Would be good also include XAS/XPS in the title so people know it immedinately, how about:

• XAS/XPS core-level spectroscopy (aiida-qe-xspec)
• or, Core-level spectroscopy for XAS/XPS (aiida-qe-xspec)

[PNOGillespie]

Since there are fields for title and description, I would go with:

aiida-qe-xspec:
    title: Core-level spectroscopy (aiida-qe-xspec)
    description: XAS and XPS calculations with Quantum ESPRESSO.
    author: Xing Wang, Peter Gillespie
    github: https://github.com/aiidalab/aiida-qe-xspec
    github: https://github.com/aiidaplugins/aiida-qe-xspec
    documentation: https://aiida-qe-xspec.readthedocs.io/
    pip: aiida-qe-xspec

Let me know if that's acceptable, and I will update the PR.

[superstar54]

LGTM!