Revert "[Enhancement] Add form changes for Solgaleo, Lunala, & Marshadow cosmetic forms"

[DayKev]

Reverts #5344

[EscaShark]

I left some of the reasoning for this on the previous comment, but will reiterate, and include some things said in contributor-chat about it as well. (ftr Kev is authoring the PR because for some reason an easy Revert button did not appear for me; could apparently be because I do not have balance-team set on GitHub currently)

In my opinion, it is at best unnecessary and at worst detrimental to add these custom formes, for these reasons:

• They are not actually formes. These Pokémon do not transform in mainline series nor does any dex entry recognize these cosmetic changes as formes (unlike, say, Inactive Xerneas). They are really just part of move animations, no different from Mega Sharpedo with its teeth extended, Iron Valiant with its blade drawn, or Overdrive Zekrom. This might be intentional, but Madness said in the original PR I quote "These are custom form change methods since the Z-Moves these forms are usually attached to are not planned to be implemented." This seems to imply that he believes these are actual formes that are simply tied to Z-Moves, erroneously--on both the signature Z-Moves of these mons and the non-Z signature moves, these are only part of a move animation, not an actual forme.
• I recognize that assets for these "formes" exist. However, that does not give us an obligation to use them. If I (attempt to) add a Mega Sharpedo with extended teeth to the sprite database tomorrow, the contributor team is not forced to find a use case for it. There's not even a fallacy of sunk costs since to my knowledge the Radiant Sun, Full Moon and Zenith sprites aren't even made by the Rogue team ourselves. It makes more sense to remove an unnecessary asset than find a confusing, custom use case for it.
• All three "formes" do not have animations, and Zenith does not have shiny variants. It is aesthetically unpleasing to go from an animated sprite to a non-animated one in the middle of a battle. In Zenith's case it's more egregious: transforming would remove your visual shininess. These are in my opinion disqualifying for custom formes meant to be cosmetics.
• It is also aesthetically weird for Lunala to remain in an awkward "charging up Moongeist Beam" forme all the time after using it once. One would expect a purely cosmetic change to at least look good. A possibility to immediately revert once the move animation is finished was brought up, but this would slow gameplay for these three mons down significantly (needing two transformations every time they want to use their signature move) as well as not fixing any of the other concerns.
• unicorn_power's planned animations for Solgaleo and Lunala already refer to these "formes", so even if this is meant as something like a "Hey, we know Radiant Sun Solgaleo exists!", we already have other ways to do so.

Due to these formes not actually existing, the reasoning for adding them seemingly being erroneous to begin with, and multiple complications being brought with implementing them, I believe it is not a good idea to do this and strongly support reverting #5344.