Remove v2_ prefixes (threedigrid)

[leendertvanwolfswinkel]

content_type values should be updated according to the new table names (v2_weir -> weir, v2_pumpstation -> pump etc)

[margrietpalm]

Should be resolved in 1D migration, but lets dubble check Didn't resolve this.

@martijn-siemerink do you think it's a good idea to modify this in threedigrid?

[jpprins1]

The content_type field is exposed to the outside (so part of the public API) and therefore we cannot change this because of backwards compatibility.

[margrietpalm]

Conlcusion: no changes will be made but the usage of v2_ prefixes will be described in the documenation by @leendertvanwolfswinkel