st/mesa: minor refactoring of texture/sampler delete code

st/mesa: minor refactoring of texture/sampler delete code

Rename st_texture_free_sampler_views() to
st_delete_texture_sampler_views() to align with
st_DeleteTextureObject(), its only caller.

Move the call to st_texture_release_all_sampler_views() from
st_DeleteTextureObject() to st_delete_texture_sampler_views()
so all the sampler view clean-up code is in one place.

Reviewed-by: Neha Bhende <bhenden@vmware.com>