Delete .github/ISSUE_TEMPLATE directory (#87)

From now on, any Github repo under the Nvidia-NeMo org will use the
default Issues system / behavior
(unless overwritten i.e. in this case the individual repo would have a
.github/ISSUE_TEMPLATE folder in the repo itself, effectively overriding
the default behavior coming from the NVIDIA-NeMo/.github repo [which
establishes the default system]).

Default behavior for Issues creation now is :
- There is a Bug Report issue
- There is a Feature Request issue
- Blank issues are NOT allowed to be created

Author: pablo-garay