How to remove the line 'Prepared for submission to JCAP'?

Welcome! Just add

makeatletter
gdef@fpheader{}
makeatother

before

begin{document}

Example:

documentclass{article}
usepackage{jcappub}
title{An example title where boldmath $x=1$}
author[a]{First Author,} 
author[a,b,1]{Second Author% 
note{On leave from XXX.}} 
author[a,c]{and Third Author}
affiliation[a]{Institution,
 Street number, City, Country}
affiliation[b]{Department, University,
 Street number, City, Country}
affiliation[c]{Another University,
Street number, City, Country}
emailAdd{[email protected]} 
emailAdd{[email protected]} 
emailAdd{[email protected]}
abstract{Text, text $math$, $math$ text, text. Text, text $math$, $math$ text,
text... }
keywords{keyword one, keyword two} 
arxivnumber{1234.5678}
makeatletter
gdef@fpheader{}
makeatother
begin{document}
maketitle
section{Introduction}
end{document}