Remove HorizontalAlignment enum from graphics_types.h

It's unused, and its values kAlignLeft and kAlignRight were conflicting
with values in the CaretAlignment enum inside
LayoutBoxModelObject::LocalCaretRectForEmptyElement() when compiling
with -Wshadow

Bug: 294205
Change-Id: Iaf89021bbb79aa4e46539284c3db5ba2d2844367
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2455688Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814753}

third_party/blink/renderer/platform/graphics/graphics_types.h

@@ -162,8 +162,6 @@ enum LineJoin {
   kBevelJoin = SkPaint::kBevel_Join
 };
 
-enum HorizontalAlignment { kAlignLeft, kAlignRight, kAlignHCenter };
-
 enum TextBaseline {
   kAlphabeticTextBaseline,
   kTopTextBaseline,