Remove stale doc comment

This warning is no longer necessary after https://android-review.googlesource.com/c/platform/build/soong/+/1235631

Bug: 160208353
Change-Id: I006272e8d4c3780d7e9f452013ca9d5db3c63613
Test: none

genrule/genrule.go

@@ -83,7 +83,6 @@ type hostToolDependencyTag struct {
 
 type generatorProperties struct {
 	// The command to run on one or more input files. Cmd supports substitution of a few variables
-	// (the actual substitution is implemented in GenerateAndroidBuildActions below)
 	//
 	// Available variables for substitution:
 	//
@@ -94,9 +93,6 @@ type generatorProperties struct {
 	//  $(depfile): a file to which dependencies will be written, if the depfile property is set to true
 	//  $(genDir): the sandbox directory for this tool; contains $(out)
 	//  $$: a literal $
-	//
-	// All files used must be declared as inputs (to ensure proper up-to-date checks).
-	// Use "$(in)" directly in Cmd to ensure that all inputs used are declared.
 	Cmd *string
 
 	// Enable reading a file containing dependencies in gcc format after the command completes