Do not use CSS to hide the options nav.

This commit removes the CSS rules used to hide the options nav and filters
the BP options nav to remove the markup.

Introduces new functions:

* cbox_theme_remove_user_options_nav() - To remove the options nav on user
  pages only.

* cbox_theme_reinstate_user_options_nav() - To reenable the options nav so
  Infinity can render the nested sidebar menu.

Read the inline doc for more details.

Fixes #141.