[Tex/LaTex] How to remove the section’s title from the header

Question

In my header right now i have the title of the chapter and the title of the current section, I would like to remove the title of the section of the header and
leavejust the chapter title

used packages

\documentclass[12pt]{report}
\usepackage{array}
\usepackage[T1]{fontenc}
\usepackage[francais]{babel}
\usepackage{sectsty}
\usepackage{multicol}
\usepackage{graphicx}
\usepackage{multirow}
\usepackage{dcolumn,longtable,booktabs}
\usepackage{enumitem}
\usepackage[utf8x]{inputenc} 
\usepackage{soul}
\renewcommand{\baselinestretch}{1.5} 
\usepackage{pifont}
\usepackage{caption}
\usepackage{mathptmx}
\pagestyle{headings}
\usepackage[nottoc]{tocbibind}
\usepackage{comment}
\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
\usepackage{titlesec}
\usepackage{titletoc}
\setcounter{secnumdepth}{4}
\setcounter{tocdepth}{4}
\usepackage{fancyhdr}
\pagestyle{fancy}
\usepackage{lipsum}
\usepackage{chngcntr}\counterwithout{figure}{chapter}
\usepackage{float}
\usepackage{siunitx}
\renewcommand{\thefigure}{\arabic{figure}}
\usepackage[table]{xcolor}
\usepackage{textcomp}
\renewcommand\thesection{\Roman{section}}
\renewcommand{\headrulewidth}{0.4pt}
\sisetup{math-micro=\text{µ},text-micro=µ}
\captionsetup[table]{name=Tableau}
\sectionfont{\fontsize{12}{15}\selectfont}
\subsectionfont{\fontsize{12}{15}\selectfont}
\usepackage{tocloft}
\renewcommand{\cftsecaftersnum}{.} 
\renewcommand{\thetable}{\arabic{table}}
\usepackage[labelfont=bf]{caption}

Answer 1

In the following suggestion I will remove all the unrelated packages and code. Especially you can remove the line \pagestyle{headings} because it is overwritten by the later \pagestyle{fancy}.

For a onesided document you can remove the left header entry:

\documentclass[12pt]{report}
\usepackage[T1]{fontenc}
\usepackage[francais]{babel}
\usepackage[utf8]{inputenc}% option replaced

\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead[L]{}

\usepackage{blindtext}% only for dummy text
\begin{document}
\blinddocument
\end{document}

Or you can declare the contents of the page header:

\documentclass[12pt]{report}
\usepackage[T1]{fontenc}
\usepackage[francais]{babel}
\usepackage[utf8]{inputenc}% option replaced

\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead{
  \fancyhead[R]{\slshape\leftmark}
}

\usepackage{blindtext}% only for dummy text
\begin{document}
\blinddocument
\end{document}