WL#9170: Remove all the charset check inc files from mysql-test

Starting 8.0.0 it is not possible to compile out charsets. Hence
inc files that were used for existence of charsets can be removed. Charsets are
always compiled in and need to be checked by the tests

have_cp1250_ch.inc
have_cp1251.inc
have_cp866.inc
have_cp932.inc
have_eucjpms.inc
have_euckr.inc
have_gb18030.inc
have_gb2312.inc
have_gbk.inc
have_koi8r.inc
have_latin2_ch.inc
have_tis620.inc
have_ujis.inc
have_utf16.inc
have_utf32.inc
have_utf8mb4.inc
have_big5.inc
have_sjis.inc
have_ucs2.inc
have_utf8.inc

User Documentation
==================

Changes for test suite. No user documentation required.

FR1: The above mentioned include files SHALL be removed from mysql-test.

FR2: The reference of these include files SHALL be removed from all the MTR tests. 

From mysql 8.0 onwards, -DWITH_EXTRA_CHARSETS compile option is removed which 
implies that all the charsets will be by-default part of the product and there is 
no option of compiling out any charset.

This implementation voids the usage of have_.inc files to check for the 
existence of a particular inc file since all the charsets will always be compiled 
in.

In case any charset is not found, which is not an expected behavior the MTR test 
should fail instead of skip.