win32: remove outdated splash screen

The official Coq logo does not work as a splash screen.
Simplest fix: no splash screen.
Revert "coqc: execvp is now available even on win32"

This reverts commit 60c390951cb2d771c16758a84bf592d06769da14.

The reason is that execvp exists on windows but is "non blocking".
So coqc would detach "coqtop -compile" and make would fail trying
to step to the next target before "coqtop -compile" terminates (because
coqc did terminate already).