Completion: remove completion_{begin,end}_block()

gtk_source_completion_block_interactive() and
gtk_source_completion_unblock_interactive() can be used directly.
Completion: remove row_activated_cb()

activate_current_proposal() can be called directly.
Completion: remove len parameter of replace_current_word()

The function is called in only one place, and the parameter is -1.
Completion: simplify some utils functions

Some parameters or return values were not used.