Remove autosubst submodule.

For consistency, let's deal with autosubst in the same way as with
ssreflect. The user should install it somewhere itself.

This should be documented and possibly discussed later.

.gitmodules

-[submodule "autosubst"]
-	path = autosubst
-	url = https://github.com/tebbi/autosubst.git

_CoqProject

 -Q . ""
--R autosubst/theories Autosubst

autosubst @ 07c66946

-Subproject commit 07c669460cd84e6e0b2084e870b19f947b699ded