mm/slub: drop duplicate kernel-doc for ksize()

The implementation of ksize() was updated with kernel-doc by commit
fab0694646d7 ("mm/slab: move [__]ksize and slab_ksize() to mm/slub.c")
However, the public header still contains a kernel-doc comment
attached to the ksize() prototype.

Having documentation both in the header and next to the implementation
causes Sphinx to treat the function as being documented twice,
resulting in the warning:

WARNING: Duplicate C declaration, also defined at core-api/mm-api:521
Declaration is '.. c:function:: size_t ksize(const void *objp)'

Kernel-doc guidelines recommend keeping the documentation with the
function implementation. Therefore remove the redundant kernel-doc
block from include/linux/slab.h so that the implementation in slub.c
remains the canonical source for documentation.

No functional change.

Fixes: fab0694646d7 ("mm/slab: move [__]ksize and slab_ksize() to mm/slub.c")
Signed-off-by: Sanjay Chitroda <sanjayembeddedse@gmail.com>
Link: https://patch.msgid.link/20260226054712.3610744-1-sanjayembedded@gmail.com
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>