MM-23261 plugin stderr debug logs (#14166)

* explicitly assert panic as error log

* Revert "[MM-18150] plugin panic trace should not be lost (#13559)"

This reverts commit 5d928b4f94, while leaving the unit tests intact
and now asserting debug logs instead.

* missing license header

mlog/stdlog.go

@@ -85,13 +85,3 @@ func (l *loggerWriter) Write(p []byte) (int, error) {
 	}
 	return len(p), nil
 }
-
-type panicLoggerWriter struct {
-	logFunc func(msg string, fields ...Field)
-}
-
-func (l *panicLoggerWriter) Write(p []byte) (int, error) {
-	trimmed := string(bytes.TrimSpace(p))
-	l.logFunc(trimmed)
-	return len(p), nil
-}