packetgen: Remove redundant declarations in the generated cxx code

IsValid() is generated for all classes with the same body;
generate the method only for top most classes.

IsValid_() is easy to get mixed up with IsValid();
rename it to Validate().

Remove GetPayloadBigEndian() as it is currently unused.

Test: m
Change-Id: Id021102e8e3d0c2e0526b242c04af1faaea166b1