[Mac] Remove unnecessary button on 'Bookmark all tabs' window

Remove unnecessary button on 'Bookmark all tabs' window which appears
before "Bookmarks Bar"
XIB Changes:
Removing the line in the following .xib file which is responsible for the appearance of empty button.
chrome/app/nibs/BookmarkAllTabs.xib

BUG=415554
R=asvitkine@chromium.org

TEST=
1) Launch chrome, open 2-3 tabs.
2) Right click on last tab and select 'Bookmark all tabs'
3) Empty button shouldn't be shown.

Review URL: https://codereview.chromium.org/583633002

Cr-Commit-Position: refs/heads/master@{#295714}

AUTHORS

@@ -402,6 +402,7 @@ ShankarGanesh K <blr.bmlab@gmail.com>
 Shez Baig <sbaig1@bloomberg.net>
 Shiliu Wang <aofdwsl@gmail.com>
 Shiliu Wang <shiliu.wang@intel.com>
+Shilpa Shri <shilpa.shri@samsung.com>
 Shouqun Liu <shouqun.liu@intel.com>
 Shreeram Kushwaha <shreeram.k@samsung.com>
 Shreyas Gopal <shreyas.g@samsung.com>

chrome/app/nibs/BookmarkAllTabs.xib

@@ -832,7 +832,6 @@
 						<reference key="object" ref="607350872"/>
 						<object class="NSMutableArray" key="children">
 							<bool key="EncodedWithXMLCoder">YES</bool>
-							<reference ref="329152418"/>
 							<reference ref="904735886"/>
 							<reference ref="792927203"/>
 							<reference ref="316831958"/>