modfile: fix Cleanup and DropTool documentation

WorkFile.Cleanup currently links to the nonexistent
WorkFile.DropRequire method. Link to WorkFile.DropUse instead, which
uses the deferred cleanup behavior described by Cleanup.

Also fix File.DropTool's doc comment to name DropTool instead of
RemoveTool.

Change-Id: If5ee772f69dab2a38acbbc8a24cc1104334be365
Reviewed-on: https://go-review.googlesource.com/c/mod/+/819820
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>

modfile/rule.go

diff --git a/modfile/rule.go b/modfile/rule.go
index 20ba825..5b2466f 100644
--- a/modfile/rule.go
+++ b/modfile/rule.go
@@ -1678,7 +1678,7 @@
 	return nil
 }
 
-// RemoveTool removes a tool directive with the given path.
+// DropTool removes a tool directive with the given path.
 // It does nothing if no such tool directive exists.
 func (f *File) DropTool(path string) error {
 	for _, t := range f.Tool {

modfile/work.go

diff --git a/modfile/work.go b/modfile/work.go
index 09df5ea..92db618 100644
--- a/modfile/work.go
+++ b/modfile/work.go
@@ -84,7 +84,7 @@
 }
 
 // Cleanup cleans up the file f after any edit operations.
-// To avoid quadratic behavior, modifications like [WorkFile.DropRequire]
+// To avoid quadratic behavior, modifications like [WorkFile.DropUse]
 // clear the entry but do not remove it from the slice.
 // Cleanup cleans out all the cleared entries.
 func (f *WorkFile) Cleanup() {