cmd/tip: delete

The cmd/tip program moved to x/build (along with its history) in:

  https://go.googlesource.com/build/+/63986c177d1ff5d2629840f6b00c445f1cb932bf

(There's no associated Gerrit CL; the merge commit of x/build's old
HEAD + a git-filter-branch of x/tools's cmd/tip was pushed directly to
Gerrit's git server, without creating Gerrit CLs for review for each
commit in its history)

Updates golang/go#29981

Change-Id: I16b9b1b0079e3d7b6851cb3e7322a878ece73e23
Reviewed-on: https://go-review.googlesource.com/c/160817
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>