`to_set` -> `xsection`

[affeldt-aist]

https://github.com/math-comp/analysis/blob/53a6bff77bd285d33cb91094cfbee9e092837f34/theories/topology.v#L4000

the to_set notation is declared here and also in other files but is redundant with the xsection definition (modulo the Prop/bool difference)