internal/impl: drop legacy prefix for some files

Both legacy_unknown.go and legacy_extension.go deal with the
current data structure for unknown fields and extensions.
While the current data structure is sub-optimial,
strictly speaking, they are not "legacy" since they are very much
used by the v2 API as well.

Change-Id: I3852dba0dcebaee00bbeb94d9936cd759f774f0a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/178477
Reviewed-by: Damien Neil <dneil@google.com>