Remove old post_processing example

[IceSentry]

Objective

• The old post processing example doesn't use the actual post processing features of bevy. It also has some issues with resizing. It's also causing some confusion for people because accessing the prepass textures from it is not easy.
• There's already a render to texture example
• At this point, it's mostly obsolete since the post_process_pass example is more complete and shows the recommended way to do post processing in bevy. It's a bit more complicated, but it's well documented and I'm working on simplifying it even more

Solution

• Remove the old post_processing example
• Rename post_process_pass to post_processing

Reviewer Notes

The diff is really noisy because of the rename, but I didn't change any code in the example.

[nicopap]

LGTM. For future reference: I think it's possible to do the change in two commit to avoid the git mixup.

[mockersf]

Could you also rename the shader post_process_pass.wgsl to the new example name?

[IceSentry]

Done!