print default --key_derivations only once
Since the default value is also embedded in the help string it appears twice in the output from `grin wallet help`.  This PR just removes the default from the help string.
@@ -201,7 +201,7 @@ fn main() {
.arg(Arg::with_name("key_derivations")
.help("The number of keys possiblities to search for each output. \
Ideally, set this to a number greater than the number of outputs \
- you believe should belong to this seed/password. (Default 1000)")
+ you believe should belong to this seed/password.")
.short("k")
.long("key_derivations")
.default_value("1000")
