Remove x/y coordinates in mviri_l1b_fiduceo_nc

[sfinkens]

Remove x/y coordinates from mviri_l1b_fiduceo_nc datasets as they only contain image rows/columns and prevent satpy from adding projection coordinates (based on the area definition).

• Tests added
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #1481 (636eb43) into master (9101690) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1481   +/-   ##
=======================================
  Coverage   90.70%   90.70%           
=======================================
  Files         241      241           
  Lines       34758    34761    +3     
=======================================
+ Hits        31526    31529    +3     
  Misses       3232     3232           

Flag	Coverage Δ
behaviourtests	4.47% <0.00%> (-0.01%)	⬇️
unittests	91.17% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
...py/tests/reader_tests/test_mviri_l1b_fiduceo_nc.py	100.00% <ø> (ø)
satpy/readers/mviri_l1b_fiduceo_nc.py	100.00% <100.00%> (ø)

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[mraspaud]

LGTM!