csharpcomp*, csharpexec*: Remove support for pnet.
* m4/csharpcomp.m4 (gt_CSHARPCOMP): Don't test for cscc. Don't set
HAVE_CSCC.
* build-aux/csharpcomp.sh.in (options_cscc): Remove variable.
Don't test HAVE_CSCC.
* lib/csharpcomp.c (compile_csharp_using_pnet): Remove function.
(compile_csharp_class): Don't invoke it.
* m4/csharpexec.m4 (gt_CSHARPEXEC): Don't test for ilrun. Don't set
HAVE_ILRUN.
* build-aux/csharpexec.sh.in (options_ilrun): Remove variable.
Don't test HAVE_ILRUN.
* lib/csharpexec.c (execute_csharp_using_pnet): Remove function.
(execute_csharp_program): Don't invoke it.
* m4/csharp.m4 (gt_CSHARP_CHOICE): Don't recognize --enable-csharp=pnet
any more.