"Factor" and "Texture" should not have separate headings

[emackey]

Pointed out by @lexaknyazev in #1825 (comment)

The KHR_materials_transmission and KHR_materials_diffuse_transmission extensions both have section headings that separate "Factor" from "Texture". This distinction is purely mechanical in glTF, it's a choice between uniform and per-texel settings, and does not affect how a given PBR parameter is used in the material. For example:

https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_transmission#transmissionfactor
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_transmission#transmissiontexture

https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissionfactor
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissioncolorfactor
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissiontexture
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission#diffusetransmissioncolortexture

Ideally we should not use "Factor" or "Texture" in a section heading. The section heading should be just the name of the PBR parameter itself, such as "Transmission", "Diffuse Transmission", and "Diffuse Transmission Color".