remove clang pinning functions (#2326)

Summary:
bypass-github-pytorch-ci-checks

X-link: https://github.com/facebook/ocamlrep/pull/76

X-link: https://github.com/facebook/buck2/pull/598

Pull Request resolved: https://github.com/pytorch/executorch/pull/2326

Removing clang pinning functions because it's replaced by config modifiers, e.g. `fbcode/scripts/zpak/examples/PACKAGE`
```
load("fbcode_macros//build_defs:platform_modifiers.bzl", "platform_modifiers")
load(
    "fbcode//buck2/cfg/experimental/set_cfg_modifiers.bzl",
    "set_cfg_modifiers",
)

modifiers = []

modifiers += platform_modifiers.fbcode_platform("platform010-compat")

modifiers += platform_modifiers.fbcode_clang("15")

set_cfg_modifiers(cfg_modifiers = modifiers)
```

Reviewed By: dmm-fb

Differential Revision: D54681702

fbshipit-source-id: 4562f0381fddd566c29a99f4f18012967f9e5e02