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

m4/curl-compilers.m4

@@ -48,7 +48,6 @@ AC_DEFUN([CURL_CHECK_COMPILER], [
   CURL_CHECK_COMPILER_INTEL_C
   CURL_CHECK_COMPILER_CLANG
   CURL_CHECK_COMPILER_GNU_C
-  CURL_CHECK_COMPILER_LCC
   CURL_CHECK_COMPILER_SGI_MIPSPRO_C
   CURL_CHECK_COMPILER_SGI_MIPS_C
   CURL_CHECK_COMPILER_SUNPRO_C
@@ -253,26 +252,6 @@ AC_DEFUN([CURL_CHECK_COMPILER_INTEL_C], [
 ])
 
 
-dnl CURL_CHECK_COMPILER_LCC
-dnl -------------------------------------------------
-dnl Verify if compiler being used is LCC.
-
-AC_DEFUN([CURL_CHECK_COMPILER_LCC], [
-  AC_MSG_CHECKING([if compiler is LCC])
-  CURL_CHECK_DEF([__LCC__], [], [silent])
-  if test "$curl_cv_have_def___LCC__" = "yes"; then
-    AC_MSG_RESULT([yes])
-    compiler_id="LCC"
-    flags_dbg_yes="-g"
-    flags_opt_all=""
-    flags_opt_yes=""
-    flags_opt_off=""
-  else
-    AC_MSG_RESULT([no])
-  fi
-])
-
-
 dnl CURL_CHECK_COMPILER_SGI_MIPS_C
 dnl -------------------------------------------------
 dnl Verify if compiler being used is SGI MIPS C.