Removed duplicate items on table of contents

Removed duplicate, broken item in 'in this document' on Performance Tips (http://developer.android.com/traning/articles/perf-tips.html

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Change-Id: I499d9bb314fa471e6d2c0590f59c2f71077c00b8
Signed-off-by: Taeho Kim <jyte82@gmail.com>