Remove old and unused fixture.

[jkmarx]

Resolves #3333

https://github.com/refinery-platform/refinery-platform/commits/develop/refinery/core/fixtures/default-pages.json

[codecov]

Codecov Report

Merging #3402 into develop will decrease coverage by 0.12%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #3402      +/-   ##
===========================================
- Coverage    68.71%   68.58%   -0.13%     
===========================================
  Files          371      371              
  Lines        24352    24266      -86     
  Branches       942      942              
===========================================
- Hits         16733    16644      -89     
- Misses        7619     7622       +3

Impacted Files	Coverage Δ
refinery/core/serializers.py	88.35% <0%> (-4.74%)	⬇️

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