Regarding the comment: Nothing except something in your init.el can cause the effect you are having now (if you didn't start Emacs with additional command line arguments). So there must be something in your init.el that creates a new window.
Here is something I put together to (hopefully) find what causes the window splitting.
First, add an advice to
split-window-right. The lambda will be called before
split-window-right was called. The lambda will print out the backtrace frame
(advice-add 'split-window-right :before (lambda (&rest _) (print (backtrace-frames))))
Then to test it I defined a function that calls
(defun some-function () (split-window-right))
Calling it prints this:
((t backtrace-frames nil nil)
(t split-window-right nil nil)
(t some-function nil nil)
(eval-print-last-sexp nil nil)
You can find
some-function is right before
This may solve your problem, may not. But I think it worth giving a try : )