SMT2 frontend: renaming let bindings is no longer necessary

[kroening]

The solver backends no longer require that the symbols bound by let
expressions are unique.

• 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.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• 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.

[codecov]

Codecov Report

Merging #6179 (26d25a4) into develop (de99e34) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6179   +/-   ##
========================================
  Coverage    74.91%   74.91%           
========================================
  Files         1450     1450           
  Lines       158481   158489    +8     
========================================
+ Hits        118723   118730    +7     
- Misses       39758    39759    +1     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_parser.cpp	77.64% <100.00%> (+0.18%)	⬆️
src/ansi-c/c_typecheck_expr.cpp	60.08% <0.00%> (-0.05%)	⬇️
src/solvers/smt2/smt2_parser.h	100.00% <0.00%> (ø)

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