remove three deprecated goto_instructiont methods

[kroening]

This removes three deprecated methods from goto_programt::instructiont and goto_programt. Their replacement is straight-forward. The remaining users are replaced accordingly.

• 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

Patch coverage: 80.00% and project coverage change: -0.08 ⚠️

Comparison is base (19470af) 78.50% compared to head (3e8e07a) 78.43%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7566      +/-   ##
===========================================
- Coverage    78.50%   78.43%   -0.08%     
===========================================
  Files         1670     1670              
  Lines       191679   191675       -4     
===========================================
- Hits        150479   150334     -145     
- Misses       41200    41341     +141     

Impacted Files	Coverage Δ
src/cprover/cprover_parse_options.cpp	56.25% <0.00%> (ø)
src/goto-programs/goto_program.h	90.85% <ø> (+0.44%)	⬆️
src/goto-instrument/cover_basic_blocks.cpp	89.56% <100.00%> (ø)
src/goto-instrument/full_slicer.cpp	95.73% <100.00%> (ø)
src/goto-instrument/goto_program2code.cpp	69.91% <100.00%> (ø)
src/util/validate_types.cpp	36.66% <0.00%> (-56.67%)	⬇️
src/util/string_hash.cpp	9.09% <0.00%> (-36.37%)	⬇️
src/util/validate_expressions.cpp	78.12% <0.00%> (-15.63%)	⬇️
src/big-int/bigint.cc	74.73% <0.00%> (-14.27%)	⬇️
src/util/validate_helpers.h	88.88% <0.00%> (-11.12%)	⬇️
... and 4 more

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[tautschnig]

#include <util/deprecate.h> now can be/needs to be removed from goto_program.h (which in itself is great news).