Remove redundant getenv() for PGUSER, in psql help.
Previously psql obtained the value of PGUSER twice to display
a default user in its help message.
Author: Kota Miyake
Reviewed-by: Nitin Jadhav, Fujii Masao
Discussion: https://postgr.es/m/
2a3c612babdd6ed63a9d877bb575d793@oss.nttdata.com