[build] Clean up no_rtti config handling Don't assume that configs are always in the default list. Bug: fuchsia:60253 Change-Id: Ic3bc5a94c31b167bf12d0552af10712ad45052cf Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/2424825 Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>