(PUP-490) Remove partial import-support from future evaluator

The support is removed (basically a mapping of a function call
to the function "import" to a ImportExpression + removing tests.

The intermediate future parser is still using ImportExpression, so
model and related logic not changed, but marked with deprecation
and "to be removed in Puppet 4" (or whenever the intermediate parser
is removed from the code base).

The grammar/parser is modified, but this is only a removal of a comment.
(PUP-490) Remove partial support for import (the sequel)

This also removes the parts that were first marked as deprecated
as they are actually unused.
(PUP-490) Remove comment that mentions "import" from transform_calls
(PUP-490) Remove dead code after removal of ImportExpression

The AstTransformer's transform_ImportExpression will never
be called since the expression was removed from the model. This removes
the transformation as well.
Merge pull request #2148 from hlindberg/pup-490_remove-partial-import-support

(PUP-490) Remove partial import-support from future evaluator