subplot debug mode

**Description of the problem**

I have an old script which uses the undocumented **+d** modifier of **subplot -F**. I believe it worked when I wrote it, but now it doesn't make sense anymore. We need to either fix it or totally remove it, as **subplot -F** already has **+p** and **+w** for debugging purpose, right?

```
#!/usr/bin/env bash
gmt begin subplot
gmt subplot begin 2x2 -Fs5c/3c+d -Blrtb
gmt basemap -R0/10/0/10 -c
gmt basemap -R0/10/0/10 -c
gmt basemap -R0/10/0/10 -c
gmt basemap -R0/10/0/10 -c
gmt subplot end
gmt end show
```
Output: