refactor: remove train as it will be redesigned

[JoanFM]

Changes introduced
In the vision of the design changes to support training in Jina. It is better to clean the code of train features that do not match with roadmap

[codecov]

Codecov Report

Merging #2311 (f7d73c3) into master (523747e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2311      +/-   ##
==========================================
+ Coverage   90.82%   90.83%   +0.01%     
==========================================
  Files         222      222              
  Lines       11797    11757      -40     
==========================================
- Hits        10715    10680      -35     
+ Misses       1082     1077       -5     

Flag	Coverage Δ
daemon	51.21% <100.00%> (+0.06%)	⬆️
jina	91.00% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
jina/executors/compound.py	90.55% <ø> (+0.99%)	⬆️
jina/executors/decorators.py	87.24% <ø> (-0.03%)	⬇️
jina/executors/indexers/__init__.py	97.89% <ø> (ø)
jina/executors/metas.py	96.87% <ø> (ø)
jina/schemas/meta.py	100.00% <ø> (ø)
jina/executors/__init__.py	92.68% <100.00%> (+0.28%)	⬆️
jina/executors/encoders/numeric/__init__.py	100.00% <100.00%> (ø)

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[github-actions]

Latency summary

Current PR yields:

• 😶 index QPS at 1061, delta to last 3 avg.: +0%
• 😶 query QPS at 15, delta to last 3 avg.: -4%

Breakdown

Version	Index QPS	Query QPS
current	1061	15
1.1.5	1054	15
1.1.4	1059	15

Backed by latency-tracking. Further commits will update this comment.