Remove Overlay Results checkbox from Residue Search

When using the Advanced Search to search for residues, the user has the option of checking/unchecking the Overlay results checkbox. This determines whether a new search replaces an old search (no overlay), or adds to an old search (overlay). However, the checkbox is unnecessary and is confusing. The Overlay results checkbox should be removed, and the default behavior should be to add new results to old results (overlay), allowing the user to build a list of various residues/primers. If the user wishes to erase each list before searching for a new residue, they can simply click on the red X clear button.