Remove replace_all string utility

[tautschnig]

There are several reasons why we should not maintain this here:

• We do not have a single use in the code base.
• It's completely unrelated to program analysis or any other CBMC matter.
• If anyone else uses it, they are more than welcome to copy the implementation.
• It was based on MIT-licensed code copyrighted by an external author. Mixing
  licenses within individual files is questionable practice.

I believe this is still used by TG, and I would thus suggest to copy the implementation into that repository.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[hannes-steffenhagen-diffblue]

Nice

[thk123]

Is indeed used in a few places but certainly can be copied - as per new process this can be merged though and we'll fix in due course.