goto-instrument: remove --check-invariant from help output

[tautschnig]

This option was added to help output in ab65b86, but it seems this
was just a merge error as otherwise that commit had nothing to do with
such an option. No code implementing this option is known.

Fixes: #5227

• 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 #6962 (38f376f) into develop (f239925) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6962   +/-   ##
========================================
  Coverage    77.79%   77.80%           
========================================
  Files         1568     1568           
  Lines       180347   180361   +14     
========================================
+ Hits        140309   140324   +15     
+ Misses       40038    40037    -1     

Impacted Files	Coverage Δ
.../goto-instrument/goto_instrument_parse_options.cpp	70.90% <ø> (ø)
src/solvers/lowering/byte_operators.cpp	92.60% <0.00%> (-0.11%)	⬇️
src/goto-programs/remove_function_pointers.cpp	92.79% <0.00%> (+0.09%)	⬆️
src/solvers/flattening/boolbv_shift.cpp	89.65% <0.00%> (+10.34%)	⬆️

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