Re-drop -Werror=unused-parameter

It breaks gtk-doc build:
```
mockdev-scan.c: In function ‘output_object_actions’:
umockdev-scan.c:569:30: error: unused parameter ‘fp’ [-Werror=unused-parameter]
  569 | output_object_actions (FILE *fp, GType object_type)
      |                        ~~~~~~^~
cc1: all warnings being treated as errors
```

meson.build

@@ -19,7 +19,6 @@ add_project_arguments(
   '-Werror=format=2',
   #'-Werror=missing-include-dirs',
   '-Werror=unused-variable',
-  '-Werror=unused-parameter',
   '-Werror=return-type',
   '-Werror=uninitialized',
   language: 'c')