Infantry Motive Type File I/O Fixes

[Windchild292]

This PR fixes the save format for the mode, namely that it should be saving as an enum. This also removes eight year old migration code that ends up causing issues during parsing. I found this issue while working on some modernization and simplifications relating to the EntityMovementMode enum, but thought this issue should be fixed for stable.

[codecov]

Codecov Report

Merging #2372 (3589bb4) into master (b0e44da) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #2372      +/-   ##
============================================
- Coverage     10.33%   10.33%   -0.01%     
+ Complexity     3410     3409       -1     
============================================
  Files           673      673              
  Lines         94643    94640       -3     
  Branches      15915    15914       -1     
============================================
- Hits           9779     9777       -2     
+ Misses        83489    83487       -2     
- Partials       1375     1376       +1     

Impacted Files	Coverage Δ	Complexity Δ
...Q/src/mekhq/campaign/parts/InfantryMotiveType.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ekhq/campaign/parts/MissingInfantryMotiveType.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
MekHQ/src/mekhq/campaign/universe/Systems.java	41.43% <0.00%> (-0.80%)	35.00% <0.00%> (-1.00%)

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