Docs/kernel-api.rst: Remove blk-tag.c reference

After 7ca01926, legacy rq tagging was removed, so block/blk-tag.c
does not exists anymore. When generating pdfdocs, sphinx complains about
this missing file:

Error: Cannot open file ./block/blk-tag.c
Error: Cannot open file ./block/blk-tag.c
Error: Cannot open file ./block/blk-tag.c
Error: Cannot open file ./block/blk-tag.c

So remove blk-tag.c traces from kernel-api.rst file to silence these
warnings.
Signed-off-by: NMarcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/core-api/kernel-api.rst

@@ -291,12 +291,6 @@ Block Devices
 .. kernel-doc:: block/blk-lib.c
    :export:
 
-.. kernel-doc:: block/blk-tag.c
-   :export:
-
-.. kernel-doc:: block/blk-tag.c
-   :internal:
-
 .. kernel-doc:: block/blk-integrity.c
    :export: