Remove redundant blank lines to match CPython REPL

[zhihaoy]

When insert_blank_line_after_output is on,

>>> a = 'meow'
>>> a

prints two blank lines, while print(a) prints one; when insert_blank_line_after_output is off, that snippet prints one blank lines, which doesn't match CPython REPL's behavior (no extra line), while print(a) does. The two blank lines are not desirable to insert_blank_line_after_output users, and the one blank line is not desirable to non-insert_blank_line_after_output users.