Remove unused field dict_index_t::is_ngram

When MySQL 5.7 introduced fulltext parser plugins to InnoDB, it hard-coded the plugin name "ngram" to mean something special, causing the field dict_index_t::is_ngram to be set.

Because -fsanitize=undefined was issuing warnings for the assignment in row_merge_create_index() that the value for dict_index_t::is_ngram out of range for Boolean, I would prefer to remove the field altogether.

As part of this, I made an honest attempt to import plugin/fulltext from MySQL 5.7 to MariaDB 10.2. This would result in some errors due to API mismatch:

/mariadb/10.2/plugin/fulltext/ngram_parser/plugin_ngram.cc:59:14: error: use of

      undeclared identifier 'my_mbcharlen_ptr'; did you mean 'my_charlen_fix'?

                char_len = my_mbcharlen_ptr(cs, next, end);

                           ^~~~~~~~~~~~~~~~

                           my_charlen_fix

/mariadb/10.2/include/m_ctype.h:1077:6: note: 'my_charlen_fix' declared here

uint my_charlen_fix(CHARSET_INFO *cs, const char *str, const char *end)

     ^

/mariadb/10.2/plugin/fulltext/ngram_parser/plugin_ngram.cc:80:15: error: no

      member named 'position' in 'st_mysql_ftparser_boolean_info'

                        bool_info->position = start - doc;

                        ~~~~~~~~~  ^

The second one is for the field MYSQL_FTPARSER_BOOLEAN_INFO::position that was added as part of WL#6943, changing the fulltext parser plugin API and ABI in MySQL 5.7.

Also the other InnoDB fulltext parser plugin plugin/fulltext/mecab_parser/plugin_mecab.cc is filling in MYSQL_FTPARSER_BOOLEAN_INFO::position in mecab_parse().

Based on this, the InnoDB fulltext parser plugins are not directly usable with MariaDB 10.2 and we can safely remove the dict_index_t::is_ngram field and possibly revert some other WL#6943 changes.