Remove deprecated exprt::make_true

[tautschnig]

All users should use true_exprt() instead.

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[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: f56ea05).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/93252762
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[tautschnig]

@peterschrammel @thk123 @allredj This might require genuine changes in TG. Maybe those can just be done as make_true is deprecated and should thus not have a place in TG?

[thk123]

Yeah this needs a real bump bump - will create later today

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: d33be42).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/93346369
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[LAJW]

Passing Test-gen update PR: https://github.com/diffblue/test-gen/pull/2526

[tautschnig]

Do you want to merge changes in TG before this one is merged, or is this one ok to merge?

[smowton]

(it's ok to merge cbmc first)