Code contracts: cleanup get_fresh_aux_symbol wrappers

[tautschnig]

1. Consistently either use the wrappers or get_fresh_aux_symbol
   directly.
2. Remove wrappers that did not yield significant simplification.
3. Make remaining wrappers do all work that was previously done by each
   caller.

Depends on #7683 (which is the first commit).

• 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

❗ No coverage uploaded for pull request base (develop@ce4c504). Click here to learn what that means.
Patch coverage: 98.14% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #7721   +/-   ##
==========================================
  Coverage           ?   78.56%           
==========================================
  Files              ?     1685           
  Lines              ?   192848           
  Branches           ?        0           
==========================================
  Hits               ?   151510           
  Misses             ?    41338           
  Partials           ?        0           

Impacted Files	Coverage Δ
...nt/contracts/dynamic-frames/dfcc_instrument_loop.h	66.66% <ø> (ø)
...ent/contracts/dynamic-frames/dfcc_spec_functions.h	100.00% <ø> (ø)
...oto-instrument/contracts/instrument_spec_assigns.h	94.73% <ø> (ø)
src/goto-instrument/contracts/utils.h	100.00% <ø> (ø)
...ument/contracts/dynamic-frames/dfcc_instrument.cpp	83.79% <86.95%> (ø)
src/goto-instrument/contracts/contracts.cpp	94.83% <100.00%> (ø)
...trument/contracts/dynamic-frames/dfcc_cfg_info.cpp	79.00% <100.00%> (ø)
.../contracts/dynamic-frames/dfcc_instrument_loop.cpp	99.61% <100.00%> (ø)
...t/contracts/dynamic-frames/dfcc_spec_functions.cpp	92.12% <100.00%> (ø)
...instrument/contracts/dynamic-frames/dfcc_utils.cpp	98.64% <100.00%> (ø)
... and 4 more

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