Replace `strip_tags()` with `wp_strip_all_tags()` in bundled themes

[sabernhardt]

• Removes title attribute from the parent link in Twenty Ten's image template, and removes the related $post_title variable.
• Replaces strip_tags() with wp_strip_all_tags() in image templates for Twenty Eleven, Twenty Twelve and Twenty Thirteen. The title attribute remains in these themes to avoid changing the translatable strings.
• Replaces strip_tags() with wp_strip_all_tags() in ephemera widgets for Twenty Eleven and Twenty Fourteen.

Trac 62721

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