Remove `LIBC_INSTALL_PREFIX`

This matches the decision made in D99697.

It also shouldn't reintroduce the issue fixed in D99636.

The variable was originally introduced in
b22f448c but is not essential to that
change.

Once we finish adding `GnuInstallDirs` support in D100810 and D99484,
setting `CMAKE_INSTALL_LIBDIR` would also work to change the
installation directory (though for more than libc).

`GnuInstallDirs` support also brings up an issue which is avoided if
variables like `LIBC_INSTALL_PREFIX` don't exist. Because the
`GnuInstallDirs` variables can be absolute paths, it is a bit unclear
how the per-project prefixes would work: does the project-agnostic
role-specific variable (e.g. `CMAKE_INSTALL_LIBDIR`), or project-specfic
role-agnostic (e.g. `LIBC_INSTALL_PREFIX`) take priority? Each is more
specific than the other on one axis, but not the other.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D105740