Remove handling of QDOC_USE_STATIC_LIBCLANG environment
variable. The variable is not used by the Qt 6 build
system.
Change-Id: I8af68382caea2864543a3d6e6ed2922823a2e71d
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>