docs: kdoc: remove the inline states-within-a-state
The processing of inline kerneldoc comments is a state like the rest, but
it was implemented as a set of separate substates. Just remove the
substate logic and make the inline states normal ones like the rest.
INLINE_ERROR was never actually used for anything, so just take it out.
No changes to the generated output.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>