Remove forall_nodes

[tautschnig]

This was useful in the past, but with C++-11 we can use a ranged-for to
avoid the iterator altogether.

• 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 #5770 (571c265) into develop (cb9f69a) will decrease coverage by 37.46%.
The diff coverage is 35.29%.

@@             Coverage Diff              @@
##           develop    #5770       +/-   ##
============================================
- Coverage    69.67%   32.21%   -37.47%     
============================================
  Files         1246      988      -258     
  Lines       100856    83817    -17039     
============================================
- Hits         70276    26998    -43278     
- Misses       30580    56819    +26239     

Flag	Coverage Δ
cproversmt2	?
regression	?
unit	32.21% <35.29%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
src/solvers/bdd/miniBDD/miniBDD.cpp	57.26% <35.29%> (+0.05%)	⬆️
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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