chore: prune unused upstream build config and Go dependencies

[ncooke3]

Removes CMakeLists.txt, the cmake/ directory, go.mod, and go.sum. These files are remnants from the upstream BoringSSL repository that are not required for Swift Package Manager. As per the Maintenance & Sync Guide, any files not explicitly referenced in Package.swift are considered dead code and should be removed to reduce package bloat.

b/526986177

[gemini-code-assist]

Code Review

This pull request deletes several build configuration and dependency files, including the main CMakeLists.txt, CMake helper modules (OpenSSLConfig.cmake, go.cmake, and paths.cmake), and Go module files (go.mod and go.sum). There are no review comments, and I have no feedback to provide.

[ncooke3]

These changes will not affect grpc-ios.

Here is the breakdown of why this is safe:

• No CMake Usage: The grpc-ios repository does not use CMake as its build system. It lacks a CMakeLists.txt file and does not invoke CMake in any of its build scripts or configuration files (Package.swift, .podspec, etc.).
• Strictly SPM & CocoaPods: The grpc-ios repository consumes boringSSL-SwiftPM exclusively via Swift Package Manager (using Package.swift) and CocoaPods (using gRPC-Core.podspec).
• Native Compilation: Both Swift Package Manager and CocoaPods pull the boringSSL-SwiftPM source files and compile the C/C++ code natively using the platform's native build system. They completely ignore upstream build artifacts like CMakeLists.txt, go.mod, or .go scripts.

[ncooke3]

cc: @aslam9691