Since `clang-4.0` is no longer being used, it should be safe to remove
use of this diagnostic for clang. This enables using `gcc` as a workaround
for `g-ir-scanner`, as `unused-command-line-argument` is not recognized by
`gcc`.
See: https://github.com/ostreedev/ostree/pull/1871#issuecomment-503765624Closes: #1871
Approved by: jlebon