wp-env: remove mappings field

[t-hamano]

In #764, the local dev-env/themes directory is now mapped to the wp-content/themes directory to make it easier to create or check changes to themes.

This may be convenient, but in the initial setup, there are no themes, which causes the following problems:

• The frontend is blank.
• You'll get a warning that the TT5 theme is broken.

55c6c3481a08a9153af05bf7f7e6058e.mp4

The mappings field should be excluded from the default settings. Contributors who really need the mapping can do so by creating a .wp-env.override.json file like this:

{
	"mappings": {
		"wp-content/themes": "./dev-env/themes"
	}
}

[pbking]

Fair point @t-hamano