Fix string assert and dead code which caused it.

Running tools with a '--' parameter caused SkString to assert here
incorrectly. SkString::remove should allow the entire contents of a
string to be removed.

The code in the flags parser which caused this call is dead and should
be removed.

R=mtklein@google.com, reed@google.com

Author: bungeman@google.com

Review URL: https://codereview.chromium.org/453333002