go/importer: remove support for (now defunct) binary export format (reminder issue)

[griesemer]

The compiler only generates the new indexed export format, and any support for the old binary export format has been removed.

We can eliminate the support from go/importer as well.

(Before we do, double-check that the latest version of that code is ported to x/tools where it needs to survive for another 3 release cycles - until 1.14.)

[gopherbot]

Change https://golang.org/cl/143023 mentions this issue: go/importer: remove support for defunct binary export format

[gopherbot]

Change https://golang.org/cl/143024 mentions this issue: go/internal/gcimporter: rename file to better reflect new meaning

[alandonovan]

We can't fix this bug until we've done #28260, otherwise there will be no export data format that go/types can both read and write.

[griesemer]

Moving to 1.13 - depends on #28260.

[griesemer]

#28260 has been fixed.

Verify that we don't need the binary export format anywhere before taking action on this.

cc: @stamblerre