[cogl-material] Removes all the API made redundant by the blend strings API

[cogl-material] Removes all the API made redundant by the blend strings API

This removes the following API:
  cogl_material_set_blend_factors
  cogl_material_set_layer_combine_function
  cogl_material_set_layer_combine_arg_src
  cogl_material_set_layer_combine_arg_op

These were rather awkward to use, so since it's expected very few people are
using them at this point and it should be straight forward to switch over
to blend strings, the API is being removed before we release Clutter 1.0.