Don't list CSV as a valid extension for Translation resource

[timothyqiu]

Resolves #87187

When adding translation files in the Project Settings dialog, a .csv file is currently valid choice.

CSV is recognized by the resource importer to generate translation files, but it's not a Translation resource file itself.

Also removed an unused header include in the modified source file :P

Note: One drawback is that if someone created a custom importer that does import a CSV file as one valid Translation resource (which is not very useful), this PR also prevents the user from selecting the file directly. However, the user can still select the file by selecting the "All Files" filter.

[akien-mga]

Seems good for now, but we really need to change the way we deal with CSV for translations to remove this and many other hacks.

For example .csv files are the only "1 source to multiple Resources" imported files, which also mean that the generated .translation files can't be put in .godot/imported/, because there's no way for load("res://translations.csv") to be remapped to a single resource (as seen in #42392 (comment)).