Remove visualization:visualize:legacyHeatmapChartsLibrary from Advanced Setting

[markov00]

Describe the feature:
Following the decision to adopt Elastic-Charts as the sole library for data visualization in Kibana, and after years of developing and testing a Heatmap version compatible with previous features, we have decided to remove the setting visualization:visualize:legacyHeatmapChartsLibrary from the Advanced Settings and eliminate all associated code.

in #118338 The new heatmap implementation, using Elastic-Charts was released, and the Advanced Settings setting was created and turned on by default. In #154434 the deprecation badge was added and the setting was turned off by default

This change aims to streamline code maintenance, accelerate testing and CI processes, and reduce bundle size.

Tasks:

• Removing the setting and the related code altogether. All clusters with this setting ON will automatically use the new chart implementation with no major visible changes.

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)