[GN] Cleanup workaround in build/config/ios/rules.gni.
[GN] Cleanup workaround in build/config/ios/rules.gni.

Remove workaround in build/config/ios/rules.gni now that gn has
been rolled to include support for product_type property.

BUG=597975

Review-Url: https://codereview.chromium.org/1948753002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394116}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk