hterm: screen: simplify overwriteString return
hterm: screen: simplify overwriteString return

Nothing uses or expects this function to return anything, so stop
trying to return useful content.

Bug: chromium:654839
Bug: chromium:737454
Change-Id: Ib948ea493b063af46804092fa1940ec4a0b7f3eb
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/1539119
Reviewed-by: Vitaliy Shipitsyn <6f51537d61e46a00bf958b60dbef5b5870c2bf22@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm