[BZ #13691], Revert breakage of iconv() converter for TCVN-5712.

[BZ #13691], Revert breakage of iconv() converter for TCVN-5712.

	* iconvdata/tcvn5712-1.c (BODY for FROM_LOOP): Don't consider
	inptr and inend for must_buffer_ch.
	* wcsmbs/tst-mbsnrtowcs.c: Remove file.
	* wcsmbs/Makefile (tests): Remove tst-mbsnrtowcs.
	* stdio-common/Makefile (tests): Remove bug15.
	(bug15-ENV): Remove macro.
	* stdio-common/bug15.c: Remove, we do not support vi_VN.TCVN5712-1
	anymore.

localedata:
	* SUPPORTED: Remove vi_VN.TCVN/TCVN5712-1.
	* tst-strptime.c: Remove, we do not support vi_VN.TCVN5712-1
	anymore.
	* Makefile (LOCALES): Remove vi_VN.TCVN5712-1 from list.
	(tests): Remove tst-strptime.