Revert "[libc] Add realpath to linux entrypoints" (#215156)

Reverts llvm/llvm-project#212925, the tests assume that `/tmp` is a
real, valid directory on the system, which is not true for all
buildbots. `libc-riscv32-qemu-yocto-fullbuild-dbg` in particular seems
to have a symlink or similar from `/tmp` to `/var/volatile/tmp`. See
https://github.com/llvm/llvm-project/pull/212925#issuecomment-5234167255.

GitOrigin-RevId: efe96c53557041be11e7fb4d43e6448cc09f6062