golangci-lint: fix issues found by "unused" and "deadcode"

Except xattrSupported, this is a false positive.

$ golangci-lint run --disable-all --enable unused --enable deadcode
gocryptfs-xray/xray_main.go:24:5: `GitVersionFuse` is unused (deadcode)
var GitVersionFuse = "[GitVersionFuse not set - please compile using ./build.bash]"
    ^
tests/symlink_race/main.go:47:6: `chmodLoop` is unused (deadcode)
func chmodLoop() {
     ^
internal/readpassword/extpass_test.go:11:5: `testPw` is unused (deadcode)
var testPw = []byte("test")
    ^
tests/reverse/xattr_test.go:13:6: func `xattrSupported` is unused (unused)
func xattrSupported(path string) bool {
     ^
internal/fusefrontend_reverse/rpath.go:20:22: func `(*RootNode).abs` is unused (unused)
func (rfs *RootNode) abs(relPath string, err error) (string, error) {
                     ^
tests/matrix/matrix_test.go:310:6: `sContains` is unused (deadcode)
func sContains(haystack []string, needle string) bool {

1 files changed, 0 insertions, 2 deletions

diff --git a/internal/readpassword/extpass_test.go b/internal/readpassword/extpass_test.go
index 9a643a5..ea05893 100644
--- a/internal/readpassword/extpass_test.go
+++ b/internal/readpassword/extpass_test.go
@@ -8,8 +8,6 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
-var testPw = []byte("test")
-
 func TestMain(m *testing.M) {
 	// Shut up info output
 	tlog.Info.Enabled = false