boot: remove csexp and pp from external libs

This appeared to have snuck in at some point.

We track these libraries, along with re, spawn and uutf in both
duneboot.ml and bootstrap_info.ml. They are the vendored libraries we
use for bootstrap but not for building the other libraries.

Signed-off-by: Ali Caglayan <[email protected]>

Author: Alizter

boot/libs.ml

@@ -1,5 +1,5 @@
 open Types
-let external_libraries = [ "pp"; "unix"; "csexp"; "threads" ]
+let external_libraries = [ "unix"; "threads" ]
 
 let local_libraries =
   [ { path = "otherlibs/top-closure"