Remove some obsolete entries from .gitignore

Found while doing the previous .dot file changes and wondering what's
this "doc/*.in" stuff… it looks like these were obsoleted over the
years by not paying attention on cleanup patches:

- doc/build: obsoleted by commit 13de7d59
- doc/*.in: obsoleted by commit 2ab2b9f5
- doc/html: was always a directory since its introduction in commit
  d17e74b4, mark it as so
- devel/clean-cluster: was introduced with initial .gitignore addition
  (commit 7a47c582), but we don't actually generate it

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>