docs: No structured comments in kernel/dma/coherent.c

Don't try to extract comments from that file, since there are none to be
had and, seemingly, never have been.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/infrastructure.rst

@@ -49,9 +49,6 @@ Device Drivers Base
 Device Drivers DMA Management
 -----------------------------
 
-.. kernel-doc:: kernel/dma/coherent.c
-   :export:
-
 .. kernel-doc:: kernel/dma/mapping.c
    :export: