Any help in removing this default information will be very greatly appreciated. If it is, in fact, a programmed default then its presence frustrates the convenience of templates altogether, imo.
One cannot remove the default text of the default template(s) - that would defeat their purpose ;-)
Highlight the <Military, Muster Rolls (manuscript)> template and click the <Copy> button. A new template will be created named <Military, Muster Rolls (manuscript) (Copy)>. Highlight that and click the <Edit> button. Rename it if you wish and remove the 'NA–Washington' and 'National Archives, Washington, D.C.' parts from the template and click <OK> to save your changes. You now have a new template to always use, if you wish. Also, FYI, renaming it with a punctuation character ( like asterisk, dash, slash, ETC.) as the first character in it's name will cause it to sort and appear at the top of the list of templates.