CONTRACTS: remove redundant "::" in generated symbol names

[tautschnig]

This is just a beautification of goto programs generated for contracts
checking: get_fresh_aux_symbol will add "::" in this place already, so
we ended up with "::::" in symbol names.

• 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/
• 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 #6567 (9ba013d) into develop (bd1401f) will increase coverage by 0.02%.
The diff coverage is 87.27%.

@@             Coverage Diff             @@
##           develop    #6567      +/-   ##
===========================================
+ Coverage    75.99%   76.01%   +0.02%     
===========================================
  Files         1579     1579              
  Lines       181124   181304     +180     
===========================================
+ Hits        137643   137819     +176     
- Misses       43481    43485       +4     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/ansi-c/expr2c.cpp	65.94% <6.25%> (-0.46%)	⬇️
src/ansi-c/c_typecheck_code.cpp	80.07% <83.33%> (+0.40%)	⬆️
jbmc/src/java_bytecode/java_pointer_casts.cpp	92.50% <100.00%> (-0.84%)	⬇️
...s/variable-sensitivity/interval_abstract_value.cpp	84.61% <100.00%> (-0.28%)	⬇️
src/ansi-c/c_expr.h	100.00% <100.00%> (ø)
src/ansi-c/parser.y	79.64% <100.00%> (+0.35%)	⬆️
src/goto-instrument/contracts/assigns.cpp	96.35% <100.00%> (+0.51%)	⬆️
src/goto-instrument/contracts/assigns.h	100.00% <100.00%> (ø)
... and 11 more

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[SaswatPadhi]

Thanks