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

src/bin/psql/help.c

@@ -134,10 +134,7 @@ usage(unsigned short int pager)
 	fprintf(output, _("  -p, --port=PORT          database server port (default: \"%s\")\n"),
 			env ? env : DEF_PGPORT_STR);
 	/* Display default user */
-	env = getenv("PGUSER");
-	if (!env)
-		env = user;
-	fprintf(output, _("  -U, --username=USERNAME  database user name (default: \"%s\")\n"), env);
+	fprintf(output, _("  -U, --username=USERNAME  database user name (default: \"%s\")\n"), user);
 	fprintf(output, _("  -w, --no-password        never prompt for password\n"));
 	fprintf(output, _("  -W, --password           force password prompt (should happen automatically)\n"));