TinyMCE: remove box-shadow from the "more" and "nextpage" placeholder images,...

TinyMCE: remove box-shadow from the "more" and "nextpage" placeholder images, props avryl, see #24067
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git-svn-id: https://core.svn.wordpress.org/trunk@27197 1a063a9b-81f0-0310-95a4-ce76da25c4cd