Now that OIIO has proper `valid_file` APIs for the formats we care
about, and which take MemReaders, we can remove the code added to TIFF,
PSD, and PNG as part of 5cc8fea7e9.
Additionally, this change eliminates the recent console spew on startup
where the TIFF loader is asked to load non-TIFF files (it is based on
the ordering of the filetype array)[1]. We now make a `valid_file` check
during open to address this.
[1] `: Not a TIFF or MDI file, bad magic number 12150 (0x2f76).`
Pull Request: https://projects.blender.org/blender/blender/pulls/116826
75c71b78ba