RunCommand: drop exit_code keyword.
RunCommand: drop exit_code keyword.

At this point no code consumes it, thus drop it.

BUG=None
TEST=run_tests.sh
TEST=python lib/cros_build_lib_unittest.py

Change-Id: I185866bb19c5042a18f94919d4922acda345aaa6
Reviewed-on: https://gerrit.chromium.org/gerrit/16242
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

zhang0137/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite