build: delete support bits for obsolete Windows compilers

build: delete support bits for obsolete Windows compilers

- Pelles C: Unclear status, failed to obtain a fresh copy a few months
  ago. Possible website is HTTP-only. ~10 years ago I left this compiler
  dealing with crashes and other issues with no response on the forum
  for years. It has seen some activity in curl back in 2021.
- LCC: Last stable release in September 2002.
- Salford C: Misses winsock2 support, possibly abandoned? Last mentioned
  in 2006.
- Borland C++: We dropped Borland C++ support in 2018.
- MS Visual C++ 6.0: Released in 1998. curl already requires VS 2010
  (or possibly 2008) as a minimum.

Closes #12222