go/build: remove unused returnImports parameter

The code uses the filename suffix instead of the bool parameter to
determine what to do.

Fixes #19474.

Change-Id: Ic552a54e50194592a4b4ae7f74d3109af54e6d36
Reviewed-on: https://go-review.googlesource.com/38265
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/go/build/build.go

@@ -722,7 +722,7 @@ Found:
 			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
 		}
 
-		match, data, filename, err := ctxt.matchFile(p.Dir, name, true, allTags, &p.BinaryOnly)
+		match, data, filename, err := ctxt.matchFile(p.Dir, name, allTags, &p.BinaryOnly)
 		if err != nil {
 			badFile(err)
 			continue
@@ -1036,19 +1036,19 @@ func parseWord(data []byte) (word, rest []byte) {
 // MatchFile considers the name of the file and may use ctxt.OpenFile to
 // read some or all of the file's content.
 func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) {
-	match, _, _, err = ctxt.matchFile(dir, name, false, nil, nil)
+	match, _, _, err = ctxt.matchFile(dir, name, nil, nil)
 	return
 }
 
 // matchFile determines whether the file with the given name in the given directory
 // should be included in the package being constructed.
 // It returns the data read from the file.
-// If returnImports is true and name denotes a Go program, matchFile reads
-// until the end of the imports (and returns that data) even though it only
-// considers text until the first non-comment.
+// If name denotes a Go program, matchFile reads until the end of the
+// imports (and returns that data) even though it only considers text
+// until the first non-comment.
 // If allTags is non-nil, matchFile records any encountered build tag
 // by setting allTags[tag] = true.
-func (ctxt *Context) matchFile(dir, name string, returnImports bool, allTags map[string]bool, binaryOnly *bool) (match bool, data []byte, filename string, err error) {
+func (ctxt *Context) matchFile(dir, name string, allTags map[string]bool, binaryOnly *bool) (match bool, data []byte, filename string, err error) {
 	if strings.HasPrefix(name, "_") ||
 		strings.HasPrefix(name, ".") {
 		return