When deleting EXIF IFD tag, delete IFD data

[radarhere]

Resolves #9078

A user has found that after

from PIL import Image, ExifTags
with Image.open("Tests/images/exif_gps.jpg") as im:
  exif = im.getexif()
  exif.get_ifd(ExifTags.IFD.GPSInfo)
  del exif[ExifTags.IFD.GPSInfo]
  im.save("out.jpg", exif=exif)

the saved image still has GPS data.

This is because get_ifd() populates _ifds, and del exif[ExifTags.IFD.GPSInfo] doesn't clear it. This fixes that.