Sigil's not going to remove it, but I don't think Sigil created it in the first place. Regardless of the version. Even as far back as 1.9.30, Sigil didn't generate the:
<meta content="text/html; charset=UTF-8" http-equiv="content-type"/>
line in the NAV:
https://github.com/Sigil-Ebook/Sigil...cessor.cpp#L58
It must gotten pasted (or imported) into the epub in Sigil when the epub was created.