chore: drop the SimpleCov code coverage setup

[tas50]

What

Removes the SimpleCov code-coverage machinery from the spec suite.

• spec/spec_helper.rb: drops the if ENV["COVERAGE"] block that required simplecov, defined a "gem" profile, and called SimpleCov.start.
• .gitignore: drops the now-unused /coverage/ artifact path.

Why

The coverage switch was dead code that could not run. simplecov was not declared anywhere — not in the Gemfile, not in kitchen-vra.gemspec — so the guarded branch only ever produced a LoadError against a clean bundle:

$ COVERAGE=1 bundle exec rspec
An error occurred while loading spec_helper.

Failure/Error: require "simplecov"

LoadError:
  cannot load such file -- simplecov
# ./spec/spec_helper.rb:24:in '<top (required)>'
No examples found.

That is worth calling out on its own: the suite carried a code path depending on a gem nothing declared. Nothing set COVERAGE in CI, no minimum_coverage threshold was enforced, and no job collected or uploaded a report — so the only two outcomes were "unset, does nothing" and "set, blows up". Rather than add a development dependency to keep an unused switch alive, this removes the switch.

Documentation coverage is untouched: the YARD doc and doc_coverage Rake tasks are about undocumented methods in lib/, not code coverage, and they still run.

Verification

No references remain (grep -rin 'simplecov' . and grep -rn 'COVERAGE' ., both excluding .git, return nothing).

The YARD tasks are still listed and still run:

$ bundle exec rake -T
...
rake doc                    # Generate YARD Documentation
rake doc_coverage           # List anything in lib/ that is still undocumented
...

$ bundle exec rake doc_coverage
yard stats --list-undoc
Files:           2
Modules:         2 (    0 undocumented)
Classes:         1 (    0 undocumented)
Constants:       5 (    0 undocumented)
Attributes:      0 (    0 undocumented)
Methods:        17 (    0 undocumented)
 100.00% documented

Example count is unchanged at 85 before and after:

$ bundle exec rake test
85 examples, 0 failures

Lint is clean under Cookstyle 9.0.0 / RuboCop 1.90.0:

$ bundle exec cookstyle --chefstyle
8 files inspected, no offenses detected

Merge order

Branched from main at 05048a6 (after #92 merged), and this conflicts with neither open PR:

• fix: stop prompting for credentials on every run with cache_credentials #91 touches lib/kitchen/driver/vra.rb and spec/vra_spec.rb only — no overlap with spec/spec_helper.rb or .gitignore. Order does not matter.
• test: exercise the driver through Test Kitchen against a stubbed vRA #93 touches .gitignore and the Rakefile. Its .gitignore hunk appends .kitchen/ at the end of the file, twelve lines below the /coverage/ line removed here, and its Rakefile change is to the RSpec task definitions, which this PR does not touch at all. The hunks are far enough apart to merge cleanly in either order.