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Goma RBE only supports paths that are relative to the source dir
while build not using Goma or using old Goma backend supports both
so always use symbolic link to system Xcode on iOS.

This fix the compilation for people using Goma as Goma RBE is now
the default backend and without this change, the build would fall
back to local compilation, drastically slowing the build.

Stop using the ios_enable_relative_sdk_path and ios_use_goma_rbe
gn variables as they are now always expected to be `true`. Keep
the variable ios_use_goma_rbe to avoid breaking the bots that do
override the value but ignore it (bot set it to true anyway).

Bug: none
Change-Id: I2a3c098304594fb20491ab92100e9a4101a048db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2555117
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830616}