Move is_null_pointer to constant_exprt

[tautschnig]

Removes one of the functions from the expr_util translation unit that is supposed to be deprecated.

• Each commit message has a non-empty body, explaining why the change was made.
• 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

Attention: Patch coverage is 74.35897% with 10 lines in your changes missing coverage. Please review.

Project coverage is 78.17%. Comparing base (4ae54e6) to head (7b47e9f).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/analyses/custom_bitvector_analysis.cpp	0.00%	3 Missing ⚠️
src/goto-instrument/dump_c.cpp	0.00%	2 Missing ⚠️
src/analyses/invariant_set.cpp	0.00%	1 Missing ⚠️
src/cpp/cpp_typecheck_conversions.cpp	0.00%	1 Missing ⚠️
src/goto-instrument/goto_program2code.cpp	0.00%	1 Missing ⚠️
src/goto-programs/xml_expr.cpp	0.00%	1 Missing ⚠️
src/util/std_expr.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8445      +/-   ##
===========================================
- Coverage    78.25%   78.17%   -0.08%     
===========================================
  Files         1726     1726              
  Lines       188926   189113     +187     
  Branches     18363    18293      -70     
===========================================
- Hits        147844   147843       -1     
- Misses       41082    41270     +188     

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