remove XCTestManifests.

[kylebrowning]

Removes an unused file during tests.

Checklist

• [x] Circle CI is passing (code compiles and passes tests).
• [x] There are no breaking changes to public API.
• [x] New test cases have been added where appropriate.
• [x] All new code has been commented with doc blocks ///.
• [x] The linuxMain.swift is regenerated using swift test --generate-linuxmain

[codecov-io]

Codecov Report

Merging #59 into master will increase coverage by 0.92%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   93.73%   94.65%   +0.92%     
==========================================
  Files          24       23       -1     
  Lines        1437     1423      -14     
==========================================
  Hits         1347     1347              
+ Misses         90       76      -14

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