chore: remove unassigned splash image

[erisu]

Motivation, Context & Description

Reasons for Removing

• Removes the unassigned launch image warning.
• The image is assigned in the platform repo template, but on new project creation the image is unassigned because the prepare step removes the image from assignment. This is because config.xml does not have a splash tag by default.
• Android does not show splash screen by default, for consistency iOS should not display splash screen as well.

Other Thoughts

• IF a default splash screen is desired, IMO, it is recommended that the default image and config.xml splash settings are in our default template repo (cordova-app-hello-world).
• IF a default splash is desired, a default should exist on all core platforms which can be again configured in our default template repo.
• It is also expected that an end user would replace the splash screen so it may not be required.

Testing

• npm t
• cordova platform add
• cordova build

Checklist

• I've run the tests to see all new and existing tests pass

[codecov-commenter]

Codecov Report

Merging #873 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #873   +/-   ##
=======================================
  Coverage   74.40%   74.40%           
=======================================
  Files          13       13           
  Lines        1676     1676           
=======================================
  Hits         1247     1247           
  Misses        429      429           

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