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: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>