[Documentation] remove legacy argument from HGTConv docstring (#8207)

closes #8185

Background: #7117 replaces the previous `HGTConv` with an updated
implementation but kept the argument `group` in the docstring which
isn't used anymore. As only the documentation is touched, no further
testing is needed.

torch_geometric/nn/conv/hgt_conv.py

@@ -37,10 +37,6 @@ class HGTConv(MessagePassing):
             information.
         heads (int, optional): Number of multi-head-attentions.
             (default: :obj:`1`)
-        group (str, optional): The aggregation scheme to use for grouping node
-            embeddings generated by different relations
-            (:obj:`"sum"`, :obj:`"mean"`, :obj:`"min"`, :obj:`"max"`).
-            (default: :obj:`"sum"`)
         **kwargs (optional): Additional arguments of
             :class:`torch_geometric.nn.conv.MessagePassing`.
     """