QSvgDebug: remove typeName function and use QSvgNode::typeName instead

Both QSvgNode::typeName and typeName functions in qsvgdebug.cpp return
a string of the node's name from a QSvgNode::Type enum value.
This commit unifies this approach by removing typeName function and
using QSvgNode::typeName in its stead.

Amends e9913b70717f55efb88e3022254299c848bcb940 (6.7).

Pick-to: 6.12 6.11
Change-Id: I211cc62466e8d4a130ab990b95952eda85058588
Reviewed-by: Marc Mutz <marc.mutz@qt.io>