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/common/cmd_ubi.c b/common/cmd_ubi.c
index 64a7307..bbca389 100644
--- a/common/cmd_ubi.c
+++ b/common/cmd_ubi.c
@@ -609,5 +609,5 @@
 	"[Legends]\n"
 	" volume: charater name\n"
 	" size: KiB, MiB, GiB, and bytes\n"
-	" type: s[tatic] or d[ynamic] (default=dynamic)\n"
+	" type: s[tatic] or d[ynamic] (default=dynamic)"
 );