maintner/gostats: delete

As far as I can tell, this is a command that's not actively used.
Despite being described as a command that computes stats about
the Go project, it implements one trivial count of top modified
files in the main repository, and hasn't been otherwise touched
since its initial commit in 2017. This information can probably
be retrieved by those interested more easily than finding this
command.

Remove it as the last inactive piece in the early 2022 cleaning.
To the best of my knowledge, x/build now has only actively used
code as of this CL.

Fixes golang/go#51867.

Change-Id: I2fe691f8705969966275d239f66b8b092774d504
Reviewed-on: https://go-review.googlesource.com/c/build/+/405547
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: dmitshur