WL#13449: Deprecate the use of MYSQL_PWD environment variable in libmysql

libmysql reads the password from the OS environment variable MYSQL_PWD if no
password is supplied. 

This is a bad practice as the password is available for all processes by the
same user to see and eventually for all local OS users too.

Nowadays there are plenty of better options to doing the same: mysql config
editor and login paths, adding a temporary $HOME/.my.cnf etc. 

So it's time to call this variable deprecated.