Removed const from return types where they return by value

[NathanJPhillips]

This must have been added by mistake as it has no effect at all.

• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov-io]

Codecov Report

Merging #4911 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop    #4911   +/-   ##
========================================
  Coverage    69.19%   69.19%           
========================================
  Files         1307     1307           
  Lines       107957   107957           
========================================
  Hits         74702    74702           
  Misses       33255    33255

Impacted Files	Coverage Δ
src/util/range.h	100% <100%> (ø)	⬆️

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e813166).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/119176470