ios build: Remove system headers from HEADER_SEARCH_PATHS.

It seems like the compiler already has these in the search path
implicitly. Also, the React Native template project removed them in
545072b02. Furthermore, Greg confirmed that a brand-new iOS app (not
even using React Native) doesn't have these in any
HEADER_SEARCH_PATHS.

So, remove this item from HEADER_SEARCH_PATHS in our projects and
targets.

ios/ZulipMobile.xcodeproj/project.pbxproj

@@ -1818,7 +1818,6 @@
 				);
 				HEADER_SEARCH_PATHS = (
 					"$(inherited)",
-					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 					"$(SRCROOT)/../node_modules/react-native/React/**",
 					"$(SRCROOT)/../node_modules/react-native-vector-icons/RNVectorIconsManager",
 					"$(SRCROOT)/../node_modules/react-native-device-info/RNDeviceInfo",
@@ -1860,7 +1859,6 @@
 				COPY_PHASE_STRIP = NO;
 				HEADER_SEARCH_PATHS = (
 					"$(inherited)",
-					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 					"$(SRCROOT)/../node_modules/react-native/React/**",
 					"$(SRCROOT)/../node_modules/react-native-vector-icons/RNVectorIconsManager",
 					"$(SRCROOT)/../node_modules/react-native-device-info/RNDeviceInfo",
@@ -1906,7 +1904,6 @@
 				FRAMEWORK_SEARCH_PATHS = "$(inherited)";
 				HEADER_SEARCH_PATHS = (
 					"$(inherited)",
-					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 					"$(SRCROOT)/../node_modules/react-native/React/**",
 					"$(SRCROOT)/../node_modules/react-native-vector-icons/RNVectorIconsManager",
 					"$(SRCROOT)/../node_modules/react-native-device-info/RNDeviceInfo",
@@ -1953,7 +1950,6 @@
 				FRAMEWORK_SEARCH_PATHS = "$(inherited)";
 				HEADER_SEARCH_PATHS = (
 					"$(inherited)",
-					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 					"$(SRCROOT)/../node_modules/react-native/React/**",
 					"$(SRCROOT)/../node_modules/react-native-vector-icons/RNVectorIconsManager",
 					"$(SRCROOT)/../node_modules/react-native-device-info/RNDeviceInfo",
@@ -2036,7 +2032,6 @@
 				GCC_WARN_UNUSED_VARIABLE = YES;
 				HEADER_SEARCH_PATHS = (
 					"$(inherited)",
-					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 					"$(SRCROOT)/../node_modules/react-native/React/**",
 					"$(SRCROOT)/../node_modules/react-native-vector-icons/RNVectorIconsManager",
 					"$(SRCROOT)/../node_modules/react-native-device-info/RNDeviceInfo",
@@ -2090,7 +2085,6 @@
 				GCC_WARN_UNUSED_VARIABLE = YES;
 				HEADER_SEARCH_PATHS = (
 					"$(inherited)",
-					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 					"$(SRCROOT)/../node_modules/react-native/React/**",
 					"$(SRCROOT)/../node_modules/react-native-vector-icons/RNVectorIconsManager",
 					"$(SRCROOT)/../node_modules/react-native-device-info/RNDeviceInfo",