Simplify xcconfig code sign settings

[armcknight]

Each xcconfig sets e.g. CODE_SIGN_IDENTITY_$(TARGET_NAME) to the desired value and includes Global.xcconfig afterwards (at the end of the file in the template, with a comment about resolving the indirect variable) which then unwinds it to set CODE_SIGN_IDENTITY = $(CODE_SIGN_IDENTITY_$(TARGET_NAME)).

Each xcconfig (debug, sprint, appstore, etc) could instead directly set CODE_SIGN_IDENTITY to the desired value, in place of setting it to the indirect variable. Same for PROVISIONING_PROFILE and PROVISIONING_PROFILE_$(TARGET_NAME).