The original templated assignment operator would never have been used, as
std::enable_if expects a bool value as first template argument.  But it was also
unnecessary anyway:

(1) Assignment from VclPtr<reference_type> was (and is) covered by the
    implicitly defined copy assignment operator.

(2) Assignment from naked reference_type* was covered by the user-supplied
    constructor from reference_type* to temporary, followed by (1); it is now
    covered directly by the user-supplied assignment operator from
    reference_type*.

(3) Assignment from VclPtr<derived_type> was covered by the user-supplied,
    templated constructor from VclPtr<derived_type> to temporary, followed by
    (1); it is now covered directly by the user-supplied, templated assignment
    operator from VclPtr<derived_type>.

(4) Assignment from naked derived_type* was (and is) covered by an implicit
    pointer up-cast, followed by (2).

Change-Id: I3c7527feea72fdda76d911a42ae856c353b700b5