[license] Remove contrib/gregbook from Fuchsia mirror

Bug: 90921
Change-Id: If7fb0b91748ba7f3b985abf0f6fc8749d7103a0f
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/626770
Reviewed-by: Jay Zhuang <jayzhuang@google.com>