Unnecessary checking of h5 in nc4grp.c

[edhartnett]

Similar to #779.

[edhartnett]

Why did you close this bug? The PR is not yet merged.

[WardF]

Because it didn’t contain any meaningful detail.

[edhartnett]

From #779:

When I originally wrote nc4type.c Denis had not yet added the dispatch layer. So netCDF-4 functions from that era had to deal with it if they were called on a netCDF-3 file. This results in code snippets like this (from NC4_inq_typeid() in libsrc4/nc4type.c):

 /* Must be a netCDF-4 file. */
if (!h5)
   return NC_ENOTNC4;
This is no longer needed. Now the dispatch layer will only call these functions on netCDF-4 files. Classic files will not even cause this function to be called (and will return NC_ENOTNC4 on their own).

So I can now change these to asserts.

This is also true for nc4grp.c. Since I am dealing with these two separately, since I am putting them in different PRs, I also gave them separate issues.