Remove "show" target from regression/*/Makefile

[tautschnig]

This may have been useful to someone in the past, but a semi-interactive
(starting a new vim in a loop for each directory) approach with several
hundred files is no longer meaningful.

• 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 #5957 (3a5ddc7) into develop (867168c) will increase coverage by 0.77%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5957      +/-   ##
===========================================
+ Coverage    74.24%   75.02%   +0.77%     
===========================================
  Files         1431     1431              
  Lines       155290   155295       +5     
===========================================
+ Hits        115290   116504    +1214     
+ Misses       40000    38791    -1209     

Impacted Files	Coverage Δ
src/util/simplify_expr_int.cpp	83.92% <0.00%> (+0.32%)	⬆️
src/goto-programs/goto_program.h	91.17% <0.00%> (+0.32%)	⬆️
src/symtab2gb/symtab2gb_parse_options.cpp	60.65% <0.00%> (+1.33%)	⬆️
src/goto-programs/goto_program.cpp	66.37% <0.00%> (+2.10%)	⬆️
src/pointer-analysis/value_set.cpp	82.63% <0.00%> (+3.26%)	⬆️
.../goto-instrument/goto_instrument_parse_options.cpp	67.90% <0.00%> (+4.67%)	⬆️
src/goto-instrument/wmm/event_graph.cpp	13.58% <0.00%> (+13.58%)	⬆️
src/assembler/remove_asm.cpp	50.71% <0.00%> (+13.87%)	⬆️
src/analyses/static_analysis.cpp	47.52% <0.00%> (+14.35%)	⬆️
src/util/graph.h	97.28% <0.00%> (+14.57%)	⬆️
... and 14 more

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[martin-cs]

I'm fine with this and have not heard of a user for a long time (something makes me think I saw @peterschrammel using it in ... 2015?) but maybe we should give this one a little so that, in the unlikely event of anyone having strong feelings about it, they have time to object.