Remove Pane::GetActiveEditor

The matches for this method were mostly from 
Workspace::getActiveEditor. There ended up being only two actual uses.

hashrocket/lib/hashrocket.coffee
51:    editor = atom.workspace.activePane.getActiveEditor()

nucleus/lib/core/adapters/workspace/atom/workspace_adapter.js
38:		var editor = this.paneForPath(path).getActiveEditor();