Delete all code that supported using LLVM version < 10.

Bug: b/152339534
Change-Id: I997c478b0aca6940cdd3a121eec15827eae47574
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46728
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>