Doesn't the name of the getElementsByClassName method give you a hint that it should return not a single element but multiple elements? Because there can be many elements with the same class in the document. Read the docs more carefully.
If you're familiar with CSS, there is document.querySelectorAll method, which retrieves elements via CSS selectors.
var plusLinks = document.querySelectorAll('a.no-js')
Then you can access individual links by their numeric index:
var firstLink = plusLinks[0]
As for the class attribute (and it is class attribute, not no-js attribute), you shouldn't remove it, but set it to a new value.
firstLink.setAttribute('class', 'js')
Or:
firstLink.className = 'js'
Since you want to remove the hover effect, and the body element already has no-js class on it, you can replace the class once for the whole page:
document.body.className = 'js'
document.querySelectorAll(".no-js").forEach(({ classList }) => classList.replace("no-js", "js"));. See How to change all classname elements of specific classname.