RuboCop: Remove excludes for non-existent paths

What does this MR do and why?

This speeds up RuboCop matching files because we have less patterns.

• db/schema.rb was removed in 2022 - !96880 (merged)
• plugins/ was removed in 2021 - !55168 (merged)
• db/ci_migrate was removed in 2021 - !68710 (merged)
• BackfillIntegrationsTypeNew last use was removed 2023 - !136253 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

unset REVEAL_RUBOCOP_TODO # if set

# On this branch

# Uncached - sloow
time bundle exec rubocop -P -f q
# ignore

# Cached
time bundle exec rubocop -P -f q

real    0m29.624s
user    1m36.053s
sys     0m10.993s


# On master

# Uncached - sloow
time bundle exec rubocop -P -f q
# ignore

# Cached
time bundle exec rubocop -P -f q

real    0m39.076s
user    1m47.662s
sys     0m11.127s