Removed reference to nonexistent file in g_helix.

Removed reference to nonexistent file in g_helix.

For many years, g_helix has referred to a file called n-ahx.xvg, but
no released version of Gromacs has ever produced it. Being that its contents
are largely reproduced by do_dssp, there is no need to introduce this
functionality and thus the easiest solution is to remove the reference
to this file.

Fixes #1103.

Change-Id: I2b87eed44d9d4c174d3772a39abf24827f63202d