Remove duplicate third-party test cases.

Some of the third-party test cases in the mjsunit test suite were
originally taken from WebKit and are now fully covered by the equally
named test suite.

Mapping of test cases:
 - array-isarray.js -> test/webkit/Array-isArray.js
 - array-splice-webkit.js -> test/webkit/array-splice.js

R=machenbach@chromium.org
BUG=

Review URL: https://codereview.chromium.org/158803002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19220 ce2b1a6d-e550-0410-aec6-3dcde31c8c00