[OpenMP] Remove standalone build mode (Reapply #149878) (#182022)

This reapplies #149878

Remove all the CMake code for openmp standalone builds. Standalone
builds have been superseded by the runtimes default build (also
sometimes called the standalone runtimes build). The runtimes default
build can be thought of a standalone build with the standalone
boilerplate contained in <llvm-project>/runtimes/CMakeLists.txt. There
is no need for each runtime to contain the same boilerplate code again.

Builds still using the standalone build via
```sh
cmake -S <llvm-project>/openmp ...
```
can switch over to the runtimes default build using
```sh
cmake -S <llvm-project>/runtimes -DLLVM_ENABLE_RUNTIMES=openmp ...
```
Options that were valid for the standalone build are also valid for
default runtimes build, unless handled only in `if
(OPENMP_STANDALONE_BUILD)` regions.

The existence of both, standalone builds and runtime default builds,
easily leads to confusion such as fixed in #149871. A non-standalone
build does not mean that it is built as part of a bootstrapping-build of
LLVM inside the `runtimes/runtimes-bins` directory, nor does it mean
that the compiler is necessarily Clang. Some of the remaining code may
have been written with that assumption. However, the purpose of this
patch is to only remove the standalone build functionality.

GitOrigin-RevId: 6295b8e763503644961f9087382f80965b98d466