Remove persistence from DeepImageFeaturizer

[jkbradley]

Until Apache Spark provides complete support for persistence in 3rd-party Python libraries, we need to remove this buggy implementation of persistence. Users have exported Pipelines with DeepImageFeaturizer, expecting to be able to import them back but finding they cannot.

[codecov-io]

Codecov Report

Merging #161 into master will decrease coverage by 0.1%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   85.37%   85.27%   -0.11%     
==========================================
  Files          34       34              
  Lines        1922     1908      -14     
  Branches       41       41              
==========================================
- Hits         1641     1627      -14     
  Misses        281      281

Impacted Files	Coverage Δ
python/sparkdl/transformers/named_image.py	89.05% <100%> (-1.02%)	:arrow_down:

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[lu-wang-dl]

LGTM