Remove parser tokens DECODE_MARIADB_SYM and DECODE_ORACLE_SYM

In order to fix MDEV-27744 easier, let's change the code handling sql_mode-dependent function DECODE(), as the follows:

• remove parser tokens DECODE_MARIADB_SYM and DECODE_ORACLE_SYM handling sql_mode-dependent function DECODE()
• remove the DECODE() code from sql_yacc.yy/sql_yacc_ora.yy
• add handling of DECODE() using regular Create_func means