Remove ABI composite reducerX modifiers

[djhoese]

In a previous commit I removed the reducer classes from the AHI python
code, but did not notice they were being used by the ABI composites too.

• Tests added
• Tests passed
• Passes git diff origin/develop **/*py | flake8 --diff

[codecov-io]

Codecov Report

Merging #243 into develop will increase coverage by 0.46%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #243      +/-   ##
===========================================
+ Coverage    60.67%   61.14%   +0.46%     
===========================================
  Files          111      111              
  Lines        13673    13681       +8     
===========================================
+ Hits          8296     8365      +69     
+ Misses        5377     5316      -61

Impacted Files	Coverage Δ
satpy/resample.py	37.33% <ø> (ø)	⬆️
satpy/tests/compositor_tests/test_ahi.py	97.05% <100%> (+0.39%)	⬆️
satpy/tests/compositor_tests/test_abi.py	97.14% <100%> (+0.36%)	⬆️
satpy/composites/__init__.py	33.43% <0%> (+8.32%)	⬆️
satpy/composites/cloud_products.py	23.33% <0%> (+23.33%)	⬆️

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[coveralls]

Coverage increased (+0.5%) to 61.143% when pulling 3d2146b on davidh-ssec:bugfix-abi-reducers into cd34fb9 on pytroll:develop.