Remove infinity_exprt

[tautschnig]

This removes infinity_exprt together with all its uses, where were all about array size. Any such arrays were replaced by arrays of the maximum viable size.

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[kroening]

Do we want arrays of size 2^n-1?

[tautschnig]

Do we want arrays of size 2^n-1?

Both GCC and Clang appear to refuse arrays of such size.