[tls] Remove support for pthread tls

[ctiller]

Alternative to #31030, #31036

It's looking likely that our pthread TLS implementation is no longer necessary.
@dennycd is going to confirm.

Assuming that's the case, let's finally delete this gunk.

[ctiller]

@dennycd I'm having a heck of a time sorting out ios deployment versions in this PR so I'll probably ask for some help with that piece assuming we can move forward with this.

[dennycd]

Updating w/ deployment target change removed. TLS was introduced by Apple from XCode 8.x and up

• https://stackoverflow.com/a/29929949
• WWDC 2016 What's New in LLVM

[dennycd]

objc/iOS tests now all pass, a few summary notes below

• grpc_objc_bazel_test: This is bazel-based run on newer MacOS 12.3 Monterey node with Xcode 13.3 (Clang 13.1.6). Our iOS configuration is (target device: iPhone 11, min deployment version: 9.0). This part is consistent w/ what we know about the TLS Apple support post Xcode 8/iOS 8.x. similarly for grpc_basictests_objc_ios

• grpc_basictests_cpp_ios: Cocoapod based run on older MacOS 10.14 Mojave node with Xcode 11.3 (Clang 11.0). Our passing iOS configuration is (target device: iPhone 11, min deployment version: 10.0)

  • as tested, deployment versions below 10.0 appears not yet supporting TLS w/ thread_local

The discrepancy from # 2 likely come from running on MacOS Mojave node with older Xcode toolchain (XCode 11.3) which may not yet fully support TLS on all device targets.

Would recommend we upgrade grpc_basictests_cpp_ios to MacOS Monterey node in order to remove this configuration discrepancy (cl/475944915). ; )

[dennycd]

iOS/ObjC part of this PR lgtm.