Remove depth texture from RenderTexture struct

[M374LX]

It is created but not used, apparently.

Update: It actually breaks some of the shader examples, but could these examples have their own depth textures instead of relying on one present in the RenderTexture struct?

The broken examples are:

shaders_hybrid_render.c
shaders_shadowmap.c
shaders_view_depth.c
shaders_write_depth.c

[raysan5]

@M374LX Depth texture is required.