Remove some obsolete entries from .gitignore
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
  d17e74b, mark it as so
- devel/clean-cluster: was introduced with initial .gitignore addition
  (commit 7a47c58), but we don't actually generate it

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

ganeti/ganeti,ganeti/ganeti,onponomarev/ganeti,andir/ganeti,apyrgio/ganeti,ganeti/ganeti,dimara/ganeti,leshchevds/ganeti,leshchevds/ganeti,andir/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,grnet/snf-ganeti,leshchevds/ganeti,apyrgio/ganeti,yiannist/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,bitemyapp/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,mbakke/ganeti,onponomarev/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,dimara/ganeti