cmd/go/internal/modfetch: update comment referring to old Import function

The Import function was moved from modfetch/repo.go to modload/import.go
in vgo CL 122880, and its semantics have changed in vgo CL 123095 to do
more than just searching for a module. Both of these changes were ported
to cmd/go in CL 123576.

Delete the mention of the old Import function from the modfetch/repo.go
comment, since what it refers to does not exist anymore.

Change-Id: I6dc6984128152cf9611d30fbc4e6418e91a7641f
Reviewed-on: https://go-review.googlesource.com/c/go/+/166597


Reviewed-by: Jay Conrod <jayconrod@google.com>