Remove content tests that duplicate wpt tests.

In particular, the test coverage is provided by:

dom/nodes/Node-childNodes.html
dom/nodes/Document-createElementNS.html
dom/historical.html
dom/nodes/Document-createProcessingInstruction.html
dom/nodes/Document-doctype.html
dom/nodes/Document-importNode.html
dom/nodes/Document-createElement.html
dom/nodes/Document-getElementsByTagName.html
Merge pull request #3133 from Ms2ger/rm-contenttests

Remove content tests that duplicate wpt tests; r=abinader