github: do not allow blank issues to be filed

Changing this option disables ability to file issue
without choosing one of the templates.

When only single template was availible,
this was default behaviour.
New issue was always filled with the contents of template.

This change removes confusing "file blank issue" button.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I32d88aed88a1bdf48f68ff6c19b4e83eb381afc0
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1600


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>