[MLIR] NFC. Fix unused context warning from Parser.h

NFC. Fix unused context warning from Parser.h.

```
..include/mlir/Parser/Parser.h:39:38: warning: parameter ‘context’ set but not used [-Wunused-but-set-parameter]
   39 |     Block *parsedBlock, MLIRContext *context, Location sourceFileLoc) {
```

Differential Revision: https://reviews.llvm.org/D138192