Fix superfluous and incorrect capnp cmake exports

Some of these variables are already exported by cmake files in capnproto
in third_party. And correctly.

The problem I see in my machine is that capnproto writes the static libs
to prefix/lib64/ but hardcoded CAPNP_LIBRARIES says that they are in the
prefix/lib/ directory. This makes peloton fail to link with capnp static
libs.

Let capnp cmake files in third_party handle it correctly.

Author: pranavk

cmake/External/capnproto.cmake

@@ -44,21 +44,6 @@ if (NOT __CAPNP_INCLUDED) # guard against multiple includes
       LOG_INSTALL 1
       )
 
-    set(CAPNP_FOUND TRUE)
-    set(CAPNP_EXECUTABLE ${capnp_INSTALL}/bin/capnp)
-    set(CAPNPC_CXX_EXECUTABLE ${capnp_INSTALL}/bin/capnpc-c++)
-    set(CAPNP_INCLUDE_DIRS ${capnp_INSTALL}/include)
-    set(CAPNP_INCLUDE_DIRECTORY ${capnp_INSTALL}/include)
-    set(CAPNP_LIBRARIES
-      ${capnp_INSTALL}/lib/libcapnp-rpc.a
-      ${capnp_INSTALL}/lib/libcapnp.a
-      ${capnp_INSTALL}/lib/libkj-async.a
-      ${capnp_INSTALL}/lib/libkj.a
-      ${CMAKE_THREAD_LIBS_INIT}
-      )
-    set(CAPNP_LIBRARY_DIRS ${capnp_INSTALL}/lib)
-    set(CAPNP_EXTERNAL TRUE)
-
     list(APPEND external_project_dependencies capnp)
 
   endif()