vimrc: disable counterintuitive behaviour

- Never insert two spaces when joining two lines.
- When closing a split window do not raise all remaining split windows to equal
  size but only the next one. This is especially important for plugins that
  open and close windows to show additional information and than mess up the
  current split window setting.
- Map Enter key to always choose the selected item when the completion menu
  pops up instead of inserting a newline sometimes.
- Remove spell configuration that only sets the defaults.