[docs] Stop documenting defunct global variables reset

What does this MR do?

Removes a statement that .gitlab-ci.yml global variables can be reset as a whole on a per-job basis, because that does not actually work, and spreads confusion on how global and per-job variable settings are supposed to interact.

Related issues

Closes gitlab-runner#3320 (closed)

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards