Remove functionality for creating new author

Removed the functionality to create a new WordPress author from within
the post meta box.
Removed plugin options field for multiple authors

Removed the option field for enabling multiple authors. This capability
has not yet been added to the plugin, so support for the option is
premature.
Removed callback for 'multiple-authors'

Removed the callback for the multiple authors option field.
Removed 'multiple-authors' from options validation
Removed button to create new author

Removed the button to create a new author from the post meta box.
Removed Create Author from post meta box

Removed all the code for creating a new author from within the post
meta box.
Removed 'multiple-authors' option from plugin activation