g_helix refers to nonexistent file
g_helix claims to produce a file called n-ahx.xvg, but no distribution of Gromacs that I can find has ever produced it, despite being documented in every version since 3.0.
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.