Remove p_ci_builds_execution_configs table and Ci::BuildExecutionConfig model

Problem statement

The p_ci_builds_execution_configs table and its corresponding model app/models/ci/build_execution_config.rb are no longer needed with the transition to job definitions.

Action items

• Remove code that reads from execution configs
• Remove code that creates execution configs
• Remove the app/models/ci/build_execution_config.rb model
• Remove foreign key constraint on p_ci_builds referencing p_ci_builds_execution_configs
• Remove index on p_ci_builds for execution configs
• Remove the p_ci_builds_execution_configs table via migration
• Update any tests that reference this table or model