mktime/0: remove unnecessary length>=6 input requirement (#3162)

This check was redundant and useless since jv2tm() used to require 8
elements, and errored anyway if not enough elements were provided.
After commit bc96146, this check also prevents mktime/0 from accepting
now supported inputs with less than 8 elements.

Ref: https://github.com/jqlang/jq/pull/3070#issuecomment-2299902072

Reported-by: itchyny <itchyny@cybozu.co.jp>

tests/jq.test

@@ -1610,6 +1610,10 @@ strftime("%Y-%m-%dT%H:%M:%SZ")
 [2024,2,15]
 "2024-03-15T00:00:00Z"
 
+mktime
+[2024,8,21]
+1726876800
+
 gmtime
 1425599507
 [2015,2,5,23,51,47,4,63]