Clean up GLQL-related feature flags

Overview

Review and clean up GLQL-related feature flags that have been enabled for some time and may be ready for removal.

Feature Flags to Review

1. glql_work_items - Enabled by default for quite some time with mostly resolved performance problems, candidate for removal - DRI @kivikakk - Remove glql_work_items feature flag (always on) (!225241 - merged) • Asherah Connor • 18.10
   • Also in GLQL itself: Remove glql_work_items feature flag (always on) (glql!336 - merged) • Asherah Connor • 18.10
2. glql_es_integration - Enabled for some time with few recent bugs, candidate for removal
3. glql_load_on_click - Introduced as a safety measure during production readiness review, may need negotiation but candidate for removal

Goal

• Check whether each feature flag is fully released
• Determine if feature flags can be safely removed
• Document reasons if they cannot be removed
• Simplify codebase by removing unnecessary flags

TypeScript FF

glql_typescript is handled outside this work item and will be removed in Rollback TypeScript GLQL changes (#588881)

Reference

Feature flags overview: https://samdbeckham.gitlab.io/feature-flags/#g=group%3A%3Aknowledge