Duplicate entries in layers.json

[lonvia]

Rerunning the tile generation usually only adds new layers and leaves existing ones untouched. This is a good behaviour when you want to just regenerate a single layer. But there are some corner cases where you end up with bogus entries in the file:

• when changing the WebPrefixPath, all entries now exist with the old and new prefix
• when changing the name of the layer, the old layer name remains in the layer.json

Two possible solutions to the problem:

• when running with --execute-all always create the layers.json file from scratch
• always remove entries that do not correspond to the current prefix path