PEP 751: simplity installer expectations (#4105)

peps/pep-0751.rst

@@ -785,12 +785,7 @@ Expectations for Installers
   selected to install from).
 - Installers MUST error out if a package version lacks a way to install into the
   chosen environment.
-- Installers MUST support installing into an empty environment
-- Installers SHOULD support uninstalling all packages in a pre-existing
-  environment and then installing into it (sometimes referred to as *syncing*
-  an environment to the lock file).
-- Installers MAY support a way to install into a pre-existing environment that
-  tries to keep packages not listed in the lock file working.
+- Installers MUST support installing into an empty environment.
 
 
 Pseudo-Code