lang/python: remove +ipython flag

+ Replace +ipython functionality with +python/open-ipython-repl command.
+ Use python-shell-interpreter for eval handlers, repl and python
  version detection.
+ Removed various python-shell-* variables for ipython; they are already
  supported upstream.

modules/lang/python/autoload/python.el

@@ -60,7 +60,9 @@ started it."
                      (+python--extract-version "Pipenv " v)
                      +python-version-cache))
         (puthash (or (doom-project-root) default-directory)
-                 (+python--extract-version "Python " (car (process-lines "python" "--version")))
+                 (+python--extract-version
+                  "Python "
+                  (car (process-lines python-shell-intepreter "--version")))
                  +python-version-cache))
     (error "Python")))