Remove zngio.Trailer and dependent "zed dev" subcommands (#5034)

As of #4984, VNG no longer includes a zngio.Trailer.  Remove it and the
"zed dev dig section" and "zed dev dig trailer" commands, which depend
on it.

Author: nwt

cmd/zed/dev/dig/section/command.go

@@ -12,9 +12,7 @@ import (
 	"github.com/brimdata/zed/cmd/zed/dev"
 	_ "github.com/brimdata/zed/cmd/zed/dev/compile"
 	_ "github.com/brimdata/zed/cmd/zed/dev/dig/frames"
-	_ "github.com/brimdata/zed/cmd/zed/dev/dig/section"
 	_ "github.com/brimdata/zed/cmd/zed/dev/dig/slice"
-	_ "github.com/brimdata/zed/cmd/zed/dev/dig/trailer"
 	_ "github.com/brimdata/zed/cmd/zed/dev/vector/agg"
 	_ "github.com/brimdata/zed/cmd/zed/dev/vector/copy"
 	_ "github.com/brimdata/zed/cmd/zed/dev/vector/project"

cmd/zed/dev/dig/trailer/command.go

@@ -397,73 +397,3 @@ gracie
 0
 ===
 ```
-To see the detailed VNG structure described as ZSON, you can use the `vng`
-command like this:
-```
-zed dev dig section -Z 1 hello.vng
-```
-which provides the Zed output (comments added with explanations):
-```
-// First, all of the types of the encoded value sequence are declared
-// with null values (just one here).
-
-null ({a:string,b:string})
-
-// Then comes the root reassembly map.
-
-[
-    {
-        offset: 29,
-        length: 2 (int32)
-    }
-]
-
-// Finally comes the column assembly records.
-// (Again, only one schema in this example, so only one such record.)
-
-{
-    a: {
-        column: [
-            {
-                offset: 0,
-                length: 16 (int32)
-            }
-        ],
-        presence: [] ([{offset:int64,length:int32}])
-    },
-    b: {
-        column: [
-            {
-                offset: 16,
-                length: 13 (int32)
-            }
-        ],
-        presence: [] ([{offset:int64,length:int32}])
-    }
-}
-
-```
-The VNG trailer can be viewed with this command:
-```
-zed dev dig trailer -Z hello.vng
-```
-giving
-```
-{
-    magic: "ZNG Trailer",
-    type: "vng",
-    version: 2,
-    sections: [
-        31,
-        95
-    ],
-    meta: {
-        skew_thresh: 26214400,
-        segment_thresh: 5242880
-    } (=vng.FileMeta)
-} (=zngio.Trailer)
-```
-
-> Note finally, if there were 10MB of ZNG row data here, the reassembly section
-> would be basically the same size, with perhaps a few segmaps.  This emphasizes
-> just how small this data structure is compared to the data section.