gh-93744: Remove configure --with-cxx-main option (#95651)

Remove the "configure --with-cxx-main" build option: it didn't work
for many years. Remove the MAINCC variable from configure and
Makefile.

The MAINCC variable was added by the issue gh-42471: commit
0f48d98b740110a672b62d467af192ec160e56ba. Previously, --with-cxx-main
was named --with-cxx.

Keep CXX and LDCXXSHARED variables, even if they are no longer used
by Python build system.