Deprecate -e flag for %notebook magic

Maybe the magic did something else in the past? Now the flag is just
confusing.

I've left it in place so it will still work, but have no effect.

IPython/core/magics/basic.py

@@ -3,6 +3,7 b''
 from __future__ import print_function
 from __future__ import absolute_import
 
+import argparse
 import io
 import sys
 from pprint import pformat
@@ -548,11 +549,7 b' Currently the magic system has the following functions:""",'
     @magic_arguments.magic_arguments()
     @magic_arguments.argument(
         '-e', '--export', action='store_true', default=False,
-        help='Export IPython history as a notebook. The filename argument '
-             'is used to specify the notebook name and format. For example '
-             'a filename of notebook.ipynb will result in a notebook name '
-             'of "notebook" and a format of "json". Likewise using a ".py" '
-             'file extension will write the notebook as a Python script'
+        help=argparse.SUPPRESS
     )
     @magic_arguments.argument(
         'filename', type=unicode_type,
@@ -563,22 +560,25 b' Currently the magic system has the following functions:""",'
         """Export and convert IPython notebooks.
 
         This function can export the current IPython history to a notebook file.
-        For example, to export the history to "foo.ipynb" do "%notebook -e foo.ipynb".
-        To export the history to "foo.py" do "%notebook -e foo.py".
+        For example, to export the history to "foo.ipynb" do "%notebook foo.ipynb".
+        To export the history to "foo.py" do "%notebook foo.py".
+
+        The -e or --export flag is deprecated in IPython 5.2, and will be
+        removed in the future.
         """
         args = magic_arguments.parse_argstring(self.notebook, s)
 
         from nbformat import write, v4
-        if args.export:
-            cells = []
-            hist = list(self.shell.history_manager.get_range())
-            if(len(hist)<=1):
-                raise ValueError('History is empty, cannot export')
-            for session, execution_count, source in hist[:-1]:
-                cells.append(v4.new_code_cell(
-                    execution_count=execution_count,
-                    source=source
-                ))
-            nb = v4.new_notebook(cells=cells)
-            with io.open(args.filename, 'w', encoding='utf-8') as f:
-                write(nb, f, version=4)
+
+        cells = []
+        hist = list(self.shell.history_manager.get_range())
+        if(len(hist)<=1):
+            raise ValueError('History is empty, cannot export')
+        for session, execution_count, source in hist[:-1]:
+            cells.append(v4.new_code_cell(
+                execution_count=execution_count,
+                source=source
+            ))
+        nb = v4.new_notebook(cells=cells)
+        with io.open(args.filename, 'w', encoding='utf-8') as f:
+            write(nb, f, version=4)