Previously we set `SYMROOT` to tell Xcode where to place the build
products.  However, the "clean" operation in the Xcode "new build
system" expects that only Xcode creates the `SYMROOT` directory or
contents inside it.  Since we create that directory, "clean" fails.

We now explicitly set `CONFIGURATION_BUILD_DIR` and `TARGET_TEMP_DIR`
instead of letting Xcode compute their values from `SYMROOT`, so we no
longer need to set the latter.  Drop the now-unnecessary `SYMROOT`.

Fixes: #22550