Code contracts: match up introducing/removing parameter map modifications

[tautschnig]

Code contracts that introduce local declarations result in nested
typecheck_declaration calls. The artificial __CPROVER_return_value symbol must
then be added/removed by a single call only.

[codecov]

Codecov Report

Merging #2278 (467fa22) into develop (3d8e181) will decrease coverage by 36.62%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           develop    #2278       +/-   ##
============================================
- Coverage    68.92%   32.30%   -36.63%     
============================================
  Files         1242      984      -258     
  Lines       100885    83829    -17056     
============================================
- Hits         69535    27078    -42457     
- Misses       31350    56751    +25401     

Flag	Coverage Δ
regression	?
unit	32.30% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.cpp	42.96% <100.00%> (-38.57%)	⬇️
src/cpp/cpp_id.h	0.00% <0.00%> (-100.00%)	⬇️
src/cpp/cpp_scope.h	0.00% <0.00%> (-100.00%)	⬇️
src/cpp/cpp_token.h	0.00% <0.00%> (-100.00%)	⬇️
src/cpp/cpp_name.cpp	0.00% <0.00%> (-100.00%)	⬇️
src/cpp/cpp_util.cpp	0.00% <0.00%> (-100.00%)	⬇️
src/util/identifier.h	0.00% <0.00%> (-100.00%)	⬇️
src/xmllang/graphml.h	0.00% <0.00%> (-100.00%)	⬇️
src/cpp/cpp_is_pod.cpp	0.00% <0.00%> (-100.00%)	⬇️
src/cpp/template_map.h	0.00% <0.00%> (-100.00%)	⬇️
... and 950 more

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[tautschnig]

The affected code has been changed substantially in #6799 and the change is no longer necessary.