project: Remove unecessary calls after loading project files

document_new_file_if_non_open() and ui_focus_current_document() only make sense
when files where actually opened, but the preceeding
configuration_load_session_files() only loads the file names into the global
session_files variable, from which the files are actually opened by a later
call to configuration_open_files(). These two calls should be done after that
one and the callers generally do this already.