Remove setCaretRectangle() from InputMethodContext

As the setCaretRectangle() interface was removed from the spec,
delete the code.  As no one is using the interface,
no impact is expected.

BUG=238585
TEST=none

Review URL: https://codereview.chromium.org/111473002

git-svn-id: svn://svn.chromium.org/blink/trunk@163628 bbb929c8-8fbe-4397-9dbb-9b2b20218538