Remove redundant quality checklist from core style guide

Description

Removing the "Quality Checklist" section (before/during/after) from gitlab-translation-styleguide.md. It is entirely redundant and human-oriented:

1. Before Translation (4 items) is remove. Resource loading is enforced by system prompt step 2. "Context understood" is how LLMs work by default.
2. During Translation (5 items) is removed. The agent doesn't "check terms one by one" like a human with a dictionary. "Natural flow achieved" is how LLMs generate text.
3. After Translation (6 items) secton overlaps with system prompt step 4 validation checklist. The two additions ("culturally appropriate", "all content localized") are too vague to be verifiable by an agent.

Additionally, carrying this change over from previous MR where it was intended !54 (merged):

Core Principles (all 6 items) is redundant or not meaningful for an LLM agent. "Understand First" is how LLMs work by default. "Translate Naturally" is verbatim in the system prompt ("Reconstruct content for natural expression... Translate for meaning, not word-for-word"). "Use Approved Terminology" is enforced by system prompt steps 2 and 4. "Adapt Culturally" is vague, the language-specific guides are a better place to handle that. "Preserve Technical Content" is a less detailed duplication of the Technical Content Rules section in the same file.

Related

• Related to #20 (closed)

Type of Change

• New feature
• Bug fix
• Documentation update
• Style guide or terminology update
• Configuration change
• Other (please describe)

Changelog Entry

• Changelog entry added to CHANGELOG.md
• Entry is in the [Unreleased] section
• Used appropriate category (Added/Changed/Fixed/Deprecated/Removed/Security)
• Included MR reference at the end of the entry (e.g., (!123))