draw/llvm: don't flush in vs_llvm_delete()

draw/llvm: don't flush in vs_llvm_delete()

Fixes piglit glx-shader-sharing crash.

When shaders are shared by multiple contexts, the shader's draw context
pointer may point to a previously destroyed context.  Dereferencing the
context pointer will lead to a crash.

In this case, simply removing the flushing code avoids the crash (the
exec and sse shader paths don't flush here either).

There's a deeper issue here, however, that needs examination.  Shaders
should not keep pointers to contexts since contexts might get destroyed
at any time.

NOTE: This is a candidate for the 7.10 branch (after this has been
tested for a while).