If you need to remove the entire declaration page simply comment out the section from the tex file in overleaf -- comment out means select the section from begindeclaration to end declaration and then press ctrl+/ which would put a percent sign in front of each line of the code thus the code will not have any effect while recompiling the document
If you only need to remove the TOC entry comment out the lines stating
% \addchaptertocentry{\authorshipname} % Add the declaration to the table of contents
% \addchaptertocentry{\abstractname} % Add the abstract to the table of contents
% \addchaptertocentry{\acknowledgementname} % Add the acknowledgements to the table of
contents

Abstract page
Acknowledgement page

\addchaptertocentry
, e.g.\addchaptertocentry{\abstractname}
. Just comment out/delete those lines from the relevant parts.