Reduce entry templates for performance

[ryanve]

Current (1.5) entry templates

• entry.php
• entry-content.php
• entry-footer.php
• entry-header.php
• entry-summary.php

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Separate templates facilitate granular overrides. I rarely have needed to override these ones. Loading files hinders performance. I'd like to remove these templates in favor of a faster implementation that only uses hooks. Devs could still override via these hooks.