build: remove unsupported architectures

Currently we support arm64 architecture

Change-Id: I243531ab02b6e07cdc5e761523eb7a3379fb9cb8
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>

Ring/Ring.xcodeproj/project.pbxproj

@@ -1642,6 +1642,7 @@
 				SWIFT_OBJC_BRIDGING_HEADER = "Ring/Bridging/Ring-Bridging-Header.h";
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
 				SWIFT_VERSION = 3.0;
+				VALID_ARCHS = arm64;
 			};
 			name = Debug;
 		};
@@ -1673,6 +1674,7 @@
 				PROVISIONING_PROFILE_SPECIFIER = "";
 				SWIFT_OBJC_BRIDGING_HEADER = "Ring/Bridging/Ring-Bridging-Header.h";
 				SWIFT_VERSION = 3.0;
+				VALID_ARCHS = arm64;
 			};
 			name = Release;
 		};