cmd/go/internal/modcmd: remove documentation for auto-converting legacy formats

CL 518776 dropped the ability of 'go mod init' to convert
legacy pre-module dependency configuration files, such as automatically
transforming a Gopkg.lock to a go.mod file with similar requirements,
but some of the documentation remained.

In this CL, we remove it from the cmd/go documentation.
(CL 662675 is a companion change that removes it from the Modules
Reference page).

Updates #71537

Change-Id: Ieccc64c811c4c25a657c00e42f7362a32b5fd661
Reviewed-on: https://go-review.googlesource.com/c/go/+/662695
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>

src/cmd/go/alldocs.go

@@ -1358,8 +1358,7 @@
 //
 // Init accepts one optional argument, the module path for the new module. If the
 // module path argument is omitted, init will attempt to infer the module path
-// using import comments in .go files, vendoring tool configuration files (like
+// using import comments in .go files and the current directory (if in GOPATH).
-// Gopkg.lock), and the current directory (if in GOPATH).
 //
 // See https://golang.org/ref/mod#go-mod-init for more about 'go mod init'.
 //

src/cmd/go/internal/modcmd/init.go

@@ -22,8 +22,7 @@ must not already exist.
 
 Init accepts one optional argument, the module path for the new module. If the
 module path argument is omitted, init will attempt to infer the module path
-using import comments in .go files, vendoring tool configuration files (like
+using import comments in .go files and the current directory (if in GOPATH).
-Gopkg.lock), and the current directory (if in GOPATH).
 
 See https://golang.org/ref/mod#go-mod-init for more about 'go mod init'.
 `,

src/cmd/go/internal/modfetch/repo.go

@@ -110,9 +110,7 @@ type RevInfo struct {
 // introduced, if a path p resolves using the pre-module "go get" lookup
 // to the root of a source code repository without a go.mod file,
 // that repository is treated as if it had a go.mod in its root directory
-// declaring module path p. (The go.mod is further considered to
+// declaring module path p.
-// contain requirements corresponding to any legacy version
-// tracking format such as Gopkg.lock, vendor/vendor.conf, and so on.)
 //
 // The presentation so far ignores the fact that a source code repository
 // has many different versions of a file tree, and those versions may