General help message cleanup

Many of the help messages were not really helpful; for example, many
commands that take no arguments would not print a correct synopsis
line, but "No additional help available." which is not exactly wrong,
but not helpful either.

Commit ``Make "usage" messages more helpful.'' changed this
partially. But it also became clear that lots of "Usage" and "Help"
messages (fields "usage" and "help" in struct cmd_tbl_s respective)
were actually redundant.

This patch cleans this up - for example:

Before:
	=> help dtt
	dtt - Digital Thermometer and Thermostat

	Usage:
	dtt         - Read temperature from digital thermometer and thermostat.

After:
	=> help dtt
	dtt - Read temperature from Digital Thermometer and Thermostat

	Usage:
	dtt

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/board/sandburst/common/ppc440gx_i2c.c b/board/sandburst/common/ppc440gx_i2c.c
index c1af70c..bc88e5a 100644
--- a/board/sandburst/common/ppc440gx_i2c.c
+++ b/board/sandburst/common/ppc440gx_i2c.c
@@ -501,7 +501,7 @@
 U_BOOT_CMD(
 	iprobe1,	1,	1,	do_i2c1_probe,
 	"probe to discover valid I2C chip addresses",
-	"\n    -discover valid I2C chip addresses\n"
+	""
 );
 
 #endif	/* CONFIG_I2C_BUS1 */