A quick test says that Mend and Prettify will remove that line. It will also remove the long form: <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />.
Since the ePub spec states the UTF-8 or UTF-16 MUST be used, that declaration is not needed though I used to see it all the time even as the first line of a CSS stylesheet. It will not trigger errors from epubcheck but a question about it years back stated that it is not needed and not recommended in either ePub2 or ePub3. Minor nit is that EPUB/A (archival) MUST use UTF-8.
See
EPUB 3.3 specification (2025-Mar-27) for more information.