Backport: Don't cache images in nsimage.m (Bug#18918).

Fixes: Bug#18918

* nsterm.h (EmacsImage): Remove imageListNext, refCount, reference,
imageListSetNext, imageListNext.

* nsimage.m (ImageList, imageListNext, imageListSetNext:)
(reference): Remove.
(allocInitFromFile:): Remove searching ImageList and calling
reference (Bug#18918).
(dealloc): Remove handling if ImageList.
Fix bad bug number reference, shall be 19427.
Backport: Memory leak fix and 19133 fix.

Fixes: debbugs:19133

* nsimage.m (allocInitFromFile:): Initialize bmRep.
(dealloc): Release bmRep.
(setPixmapData): Make bmRep local so class member is not
set.
Avoid compiler warning.

* nsimage.m (setPixmapData): Rename local variable bmRep to avoid
compiler warning.