DeleteNamedImport deletes the import with a given
name and path from a parsed file, if present.
imports uses this function to delete named imports.
Fixesgolang/go#8149.
Change-Id: I84539d5627191c45f06db2d008507aee4d3b1eb2
Reviewed-on: https://go-review.googlesource.com/19951
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>