Xcode: Fix "clean" operation under the "new build system"

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. These control the build product and object file locations, respectively. Drop the now-unnecessary SYMROOT.

Fixes: #22550 (closed), #23734 (closed)