FindHDF5: fix compiler detection when HL is disabled

A valid HDF5 installation with the "high level" extensions not
configured will *fail* to be correctly detected by CMake since
commit d9e39f3f89 (FindHDF5: check that compiler wrapper can
compile a minimal program, 2020-02-10, v3.18.0-rc1~744^2~1):

```
/.../hdf5/cmake_hdf5_test.c:2:10: fatal error: 'hdf5_hl.h' file not found
```

This does not stop the configuration but does prevent flags and
libraries from being recognized.

Modules/FindHDF5.cmake

@@ -208,7 +208,6 @@ function(_HDF5_test_regular_compiler_C success version is_parallel)
     set(test_file ${scratch_directory}/cmake_hdf5_test.c)
     file(WRITE ${test_file}
       "#include <hdf5.h>\n"
-      "#include <hdf5_hl.h>\n"
       "const char* info_ver = \"INFO\" \":\" H5_VERSION;\n"
       "#ifdef H5_HAVE_PARALLEL\n"
       "const char* info_parallel = \"INFO\" \":\" \"PARALLEL\";\n"