.github/ISSUE_TEMPLATE: Update platform choices (drop azure, etc.)
.github/ISSUE_TEMPLATE: Update platform choices (drop azure, etc.)

Backend support for these was removed in bdef6ddc
(installer/pkg/config: remove old platforms, 2018-06-11,
coreos/tectonic-installer#3278).

derekhiggins/installer,derekhiggins/installer