Revert "fixed model ak135f_no_mud, which was not correct"

This reverts commit 7b76e2e58b880cf6707510e8b62c01d6b69d23c1.

Conflicts:
	doc/USER_MANUAL/bibliography.bib
	setup/constants.h.in
	src/meshfem3D/create_mass_matrices.f90
	src/meshfem3D/meshfem3D_models.f90
	src/meshfem3D/model_ak135.f90
	src/meshfem3D/model_attenuation.f90