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/+/166597Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/repo.go

@@ -160,12 +160,6 @@ type RevInfo struct {
 // To avoid version control access except when absolutely necessary,
 // Lookup does not attempt to connect to the repository itself.
 //
-// The Import function takes an import path found in source code and
-// determines which module to add to the requirement list to satisfy
-// that import. It checks successive truncations of the import path
-// to determine possible modules and stops when it finds a module
-// in which the latest version satisfies the import path.
-//
 // The ImportRepoRev function is a variant of Import which is limited
 // to code in a source code repository at a particular revision identifier
 // (usually a commit hash or source code repository tag, not necessarily