[ObjC] remove CLANG_CXX_LANGUAGE_STANDARD from podspecs

[HannahShiSFB]

This conflicts with defaults and cause build errors like firebase/firebase-ios-sdk#13989

[sampajano]

Thanks Hannah!! 😃

@paulb777 FYI :)

[sampajano]

Thanks for working on this quickly, Hannah!!

[paulb777]

Thanks, but this is a different change than I expected. I don't think this will with Firestore which expects gnu++14. Also, shouldn't gRPC compile the same way for both CocoaPods 1.15 and CocoaPods 1.16.

I expected the fix to be to add the gnu++14 setting to abseil's podspec.

[HannahShiSFB]

I'm afraid add the gnu++14 setting to abseil's podspec would cause more issues, as it can break other projects that's not setting the C++ standard which is more common and Apple's recommendation.

CocoaPods 1.15 build doesn't fail linking absl with this PR.

[paulb777]

The xcodeproj gem of CocoaPods always sets CLANG_CXX_LANGUAGE_STANDARD CocoaPods/Xcodeproj@3157b7c#diff-ff9a4d09c635f0b936a889cc315e58fb2d38bf772b1712ea795498def364621aR424.

By not overriding it, users will build differently depending upon their CocoaPods gem versions.

[paulb777]

Looks like the xcodeproj gem did another release 1.27.0 to revert the problematic Settings changes. https://github.com/CocoaPods/Xcodeproj/pull/992/files - so this change may not be necessary