libgettextpo: Remove backward compatibility with versions < 0.15 (July 2006).
* gettext-tools/libgettextpo/gettext-po.c (_GL_NO_INLINE_ERROR): Remove macro.
Don't include <error.h>, <stdbool.h>, <stdarg.h>, xerror.h, po-error.h,
format.h.
(orig_error, orig_error_at_line): Remove variables.
(void_multiline_warning, void_multiline_error): Remove functions.
(po_file_read_v2, po_file_read, po_file_write, po_error_logger,
po_message_check_format): Remove functions.
* gettext-tools/libgettextpo/gettext-po.in.h (struct po_error_handler): Update
comment.
* gettext-tools/src/po-xerror.c: Update comment.
