Remove file that should've been deleted in a previous change

This patch removes StyleBuilderCustom.h which was supposed to be removed
in https://codereview.chromium.org/98093002 but was left in by accident.

R=dstockwell
BUG=

Review URL: https://codereview.chromium.org/107293010

git-svn-id: svn://svn.chromium.org/blink/trunk@163927 bbb929c8-8fbe-4397-9dbb-9b2b20218538