[llvm] [Bazel] Remove redundant gtest_main from codegen_tests, codegen_globalisel_tests, mi_tests (PR #135316)

[via llvm-commits]

HighW4y2H3ll wrote:

Thx!! honestly... I'm not too sure how how these tests work with the CMakeLists builds... so just trying to be safe here, and hopefully not breaking things. :p

https://github.com/llvm/llvm-project/pull/135316