docs/core-api: memory-allocation: remove uses of c:func:

These are no longer needed as the documentation build will automatically
add the cross references.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>