runner: Remove stale recordTypePlaintextHandshake constant

This was a remnant of experimenting with
https://github.com/tlswg/tls13-spec/pull/1051, since abandoned.

Change-Id: I2f80f6a26a06f68d882dc84abcc8d1a9e66f9f25
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/89407
Reviewed-by: Lily Chen <chlily@google.com>
Commit-Queue: Lily Chen <chlily@google.com>
Auto-Submit: David Benjamin <davidben@google.com>

ssl/test/runner/common.go

diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 7dbde72..81217f6 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -62,12 +62,11 @@
 type recordType uint8
 
 const (
-	recordTypeChangeCipherSpec   recordType = 20
-	recordTypeAlert              recordType = 21
-	recordTypeHandshake          recordType = 22
-	recordTypeApplicationData    recordType = 23
-	recordTypePlaintextHandshake recordType = 24
-	recordTypeACK                recordType = 26
+	recordTypeChangeCipherSpec recordType = 20
+	recordTypeAlert            recordType = 21
+	recordTypeHandshake        recordType = 22
+	recordTypeApplicationData  recordType = 23
+	recordTypeACK              recordType = 26
 )
 
 // TLS handshake message types.