[Plat-10822] duplicate -lc++

[kstenerud]

Goal

Xcode 15 now warns about duplicate library flags during the linking phase.

Since c++ is now inferred by the compiler, adding -lc++ hasn't been necessary in Xcode for the past few versions.

This PR removes the manual -lc++ flag in the project and podspec.

Testing

Reran all tests to ensure that this doesn't break any builds.

[github-actions]

Bugsnag.framework binary size did not change - 713,304 bytes

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