Previously we were linking in all SIL entities
if the input was a serialized non-SIB AST, and
`-disable-sil-linking` wasn't specified. However
none of the tests appear to want this behaviour.
Stop calling `SerializedSILLoader::getAll`, and
remove the `-disable-sil-linking` option, as this
is now the default behaviour.
What these tests are testing has nothing to do with whether or not we
deserialize anything from the standard library, and not doing so saves
more than two minutes (single-threaded) on my machine from each test.
