Do not check for the presence of some SIP-18 feature imports

Features which usually issue warnings or errors if the
language.existentials, language.higherKinds and
language.implicitConversions implicits are not in scope will no longer
do so.
Removed unnecessary failing test
Merge pull request #74 from propensive/sip18-noops

Do not check for the presence of some SIP-18 feature imports