[richards] Remove deprecated extension of Richards model
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What this MR does / why does DuMux need it:
Removes the old extensions of Richards model, deprecated in release 3.6. The new implementation now properly inherits from the base Richards model.
Before you request a review from someone, make sure to revise the following points:
CHANGELOG.md
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character. Make sure there is no \ No newline at end of file
comment in "Changes" of this MR.