C# support: Remove pnet choice.

C# support: Remove pnet choice.

* DEPENDENCIES: Don't mention pnet.
* PACKAGING: Likewise.
* gettext-tools/src/format-csharp.c: Simplify comment.
* gettext-tools/src/msgfmt.cs: Remove workarounds for cscc.
* gettext-tools/src/write-csharp.c (write_csharp_code): Update comment.
* gettext-tools/doc/gettext.texi (C#): Don't mention resgen.
* gettext-tools/tests/lang-csharp: Rely on mono, not pnet.
* gettext-tools/examples/hello-csharp/INSTALL: Likewise.
* gettext-tools/examples/hello-csharp-forms/INSTALL: Likewise.
* gettext-tools/examples/hello-csharp-forms/BUGS: Remove file.