Chrome Empties: Fix width of button in TableViewIllustratedCell

Currently, the button in TableViewIllustratedCell has a leading and trailing constraint. This means the button's width is determined by the view's size, instead of the button's content's size.

Removing the leading and trailing constraints of the button allows for a more flexible sizing and is better aligned with the Chrome Empties design.

(Googlers only links)
Screenshot without constraints : https://drive.google.com/file/d/1TE00X2gZks0_FUTYzOQoQoHgXOvTo6ux/view?usp=sharing
Screenshot with constraints : https://drive.google.com/file/d/16YIpHydO0tnsxS5X5sUBMc3u2xd_UWSw/view?usp=sharing

Bug: 1098328
Change-Id: Ie858503dd221936798e4ced65664eeac67b63691
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317606Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Maxime Charland <mcharland@google.com>
Cr-Commit-Position: refs/heads/master@{#792430}

ios/chrome/browser/ui/table_view/cells/table_view_illustrated_item.mm

@@ -146,17 +146,13 @@ const CGFloat kButtonCornerRadius = 8.0;
       [_titleLabel.trailingAnchor
           constraintEqualToAnchor:stackView.trailingAnchor],
 
-      // Subtitle and button should have additional margins on both sides
+      // Subtitle should have additional margins on both sides
       [_subtitleLabel.leadingAnchor
           constraintEqualToAnchor:stackView.leadingAnchor
                          constant:kItemMargin],
       [_subtitleLabel.trailingAnchor
           constraintEqualToAnchor:stackView.trailingAnchor
                          constant:-kItemMargin],
-      [_button.leadingAnchor constraintEqualToAnchor:stackView.leadingAnchor
-                                            constant:kItemMargin],
-      [_button.trailingAnchor constraintEqualToAnchor:stackView.trailingAnchor
-                                             constant:-kItemMargin],
 
       [stackView.leadingAnchor
           constraintEqualToAnchor:self.contentView.leadingAnchor