ModernCV Remove horizontal line in sections

Here you go:

RenewDocumentCommand{section}{sm}{%
  strutsectionstyle{#2hfill}%
  parnobreakaddvspace{1ex}@afterheading
  }

The above code is the core of what is required. Now time for an MWE:
The MWE

documentclass[12pt,letterpaper]{moderncv}
moderncvstyle{classic}
moderncvcolor{blue}
usepackage{libertine}
usepackage[left=1cm,right=1cm,top=1cm,bottom=1cm]{geometry}
usepackage{kantlipsum}

makeatletter
RenewDocumentCommand{section}{sm}{%
  strutsectionstyle{#2hfill}%
  parnobreakaddvspace{1ex}@afterheading
  }

RenewDocumentCommand{subsection}{sm}{
  strutsubsectionstyle{#2}{color{color1}{subsectionfill}}%
  parnobreakaddvspace{0.5ex}@afterheading}
%
makeatother

name{SUBHAM}{SONI}
address{ABC}{XYZ}{PQR}
phone[mobile]{1234567}

begin{document}
makecvtitle
section{CAREER OBJECTIVE}
kant[1]
end{document} 

It largely depends on the style you are using (and on the version of moderncv) which is the reason why a MWE would be good.

Assuming you are using the latest version of moderncv and the classic or casual theme you can put

makeatletter
RenewDocumentCommand{section}{sm}{%
  paraddvspace{2.5ex}%
  phantomsection{}% reset the anchor for hyperrefs
  addcontentsline{toc}{section}{#2}%
  parbox[t]{hintscolumnwidth}{strutraggedleftraisebox{baseletterheight}{rule{hintscolumnwidth}{0ex}}}%
  hspace{separatorcolumnwidth}%
  parbox[t]{maincolumnwidth}{strutsectionstyle{#2}}%
  parnobreakaddvspace{1ex}@afterheading}% to avoid a pagebreak after the heading
makeatother

in your preamble (before begin{document}) to get sections with an invisible bar or

makeatletter
RenewDocumentCommand{section}{sm}{%
  paraddvspace{2.5ex}%
  phantomsection{}% reset the anchor for hyperrefs
  addcontentsline{toc}{section}{#2}%
  {strutsectionstyle{#2}}%
  parnobreakaddvspace{1ex}@afterheading}% to avoid a pagebreak after the heading
makeatother

for removing the bar completely.