Use default iOS SDK for CocoaPods project (#8603)

This matches the default for Runner.xcodeproj and fixes a linker warning
when the two projects use different values.

packages/flutter_tools/templates/create/ios.tmpl/Pods/Pods.xcodeproj/project.pbxproj

@@ -21,7 +21,7 @@
 		93A4A3777CF96A4AAC1D13BA6DCCEA73 /* Podfile */ = {isa = PBXFileReference; explicitFileType = text.script.ruby; includeInIndex = 1; name = Podfile; path = ../Podfile; sourceTree = SOURCE_ROOT; xcLanguageSpecificationIdentifier = xcode.lang.ruby; };
 		A1153EC5FE504BE5C724E20E9B411E90 /* Pods-Runner-frameworks.sh */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.script.sh; path = "Pods-Runner-frameworks.sh"; sourceTree = "<group>"; };
 		A84633CB40E489EF4DD77D43434D3024 /* Pods-Runner-resources.sh */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.script.sh; path = "Pods-Runner-resources.sh"; sourceTree = "<group>"; };
-		CEC22C73C1608DFA5D5D78BDCB218219 /* Foundation.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = Foundation.framework; path = Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS9.3.sdk/System/Library/Frameworks/Foundation.framework; sourceTree = DEVELOPER_DIR; };
+		CEC22C73C1608DFA5D5D78BDCB218219 /* Foundation.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = Foundation.framework; path = Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/Foundation.framework; sourceTree = DEVELOPER_DIR; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -158,7 +158,6 @@
 				DEBUG_INFORMATION_FORMAT = dwarf;
 				ENABLE_STRICT_OBJC_MSGSEND = YES;
 				GCC_NO_COMMON_BLOCKS = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 9.3;
 				MACH_O_TYPE = staticlib;
 				MTL_ENABLE_DEBUG_INFO = YES;
 				OTHER_LDFLAGS = "";
@@ -259,7 +258,6 @@
 				DEBUG_INFORMATION_FORMAT = "dwarf-with-dsym";
 				ENABLE_STRICT_OBJC_MSGSEND = YES;
 				GCC_NO_COMMON_BLOCKS = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 9.3;
 				MACH_O_TYPE = staticlib;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				OTHER_LDFLAGS = "";