Remove unnecessary HDF5_ROOT ifdef

[milljm]

Cmake seems to be ignoring it, and HDF5 will now be made available within environment's path (during a Conda build anyway...).

Cmake warning:

  command to set the policy and suppress this warning.

  Environment variable HDF5_ROOT is set to:

    /opt/civet/local/okapi/hdf5

  For compatibility, CMake is ignoring the variable.