Revert effective removal of KUrlPixmapProvider from API

Setting KF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x054200 for the build of
KIO results in KCOMPLETION_ENABLE_DEPRECATED_SINCE(5, 66) being always
false, so any build of KIO will not longer export the KUrlPixmapProvider
class, which breaks ABI of course.

KCOMPLETION_DISABLE_DEPRECATED_BEFORE_AND_AT overrides the KF group
settings, so KIO itself will again continue to export the KUrlPixmapProvider
class. Unless KCompletion was built itself with
EXCLUDE_DEPRECATED_BEFORE_AND_AT>=5.66.0, as in that case its generated
export header will ensure KCOMPLETION_DISABLE_DEPRECATED_BEFORE_AND_AT
is auto-raised to the matching level, whatever we pass in from outside
ourselves.
ensure