Drop binary format for memory snapshot.

[trivialfis]

• Remove saving binary snapshot. Loading is preserved for now.

[trivialfis]

Note: JSON snapshot was flipped on as default in 1.3.

[codecov-io]

Codecov Report

Merging #6513 (fcd8232) into master (0e97d97) will increase coverage by 80.38%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #6513       +/-   ##
===========================================
+ Coverage        0   80.38%   +80.38%     
===========================================
  Files           0       13       +13     
  Lines           0     3559     +3559     
===========================================
+ Hits            0     2861     +2861     
- Misses          0      698      +698     

Impacted Files	Coverage Δ
python-package/xgboost/data.py	56.96% <0.00%> (ø)
python-package/xgboost/sklearn.py	89.61% <0.00%> (ø)
python-package/xgboost/rabit.py	78.57% <0.00%> (ø)
python-package/xgboost/dask.py	81.78% <0.00%> (ø)
python-package/xgboost/core.py	81.05% <0.00%> (ø)
python-package/xgboost/compat.py	73.33% <0.00%> (ø)
python-package/xgboost/__init__.py	89.47% <0.00%> (ø)
python-package/xgboost/plotting.py	73.52% <0.00%> (ø)
python-package/xgboost/config.py	100.00% <0.00%> (ø)
python-package/xgboost/libpath.py	55.55% <0.00%> (ø)
... and 3 more

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