qdoc: Remove duplicate 'const' from function signatures in .index

When generating a function signature used in index files (and webxml
output), omit adding the 'const' attribute for const functions as
FunctionNode::signature() has already added it.

Task-number: QTBUG-66528
Change-Id: I7074034d74506d25375603f39b4c2763bd732e4f
Reviewed-by: Martin Smith <martin.smith@qt.io>