cmake: Revise external capnp search logic

No change in behavior, but the comments needed updating, so I updated them.
Then I noticed a never-true if() case and removed it.

c++/src/CMakeLists.txt

@@ -4,17 +4,16 @@
 if(BUILD_TESTING)
   include(CTest)
 
-  if (EXTERNAL_CAPNP)
+  if(EXTERNAL_CAPNP)
     # Setup CAPNP_GENERATE_CPP for compiling test schemas
     find_package(CapnProto CONFIG QUIET)
     if (NOT CapnProto_FOUND)
-      # Try and find the executables from an autotools-based installation
-      # Setup paths to the schema compiler for generating ${test_capnp_files}
-      if(NOT EXTERNAL_CAPNP AND NOT CAPNP_LITE)
-        set(CAPNP_EXECUTABLE $<TARGET_FILE:capnp_tool>)
-        set(CAPNPC_CXX_EXECUTABLE $<TARGET_FILE:capnpc_cpp>)
-      else()
-        # Allow paths to tools to be set with either environment variables or find_program()
+      # No working installation of Cap'n Proto found, so fall back to searching the environment.
+      #
+      # We search for the external capnp compiler binaries via $CAPNP, $CAPNPC_CXX, and
+      # find_program(). find_program() will use various paths in its search, among them
+      # ${CMAKE_PREFIX_PATH}/bin and $PATH.
+
       if (NOT CAPNP_EXECUTABLE)
         if (DEFINED ENV{CAPNP})
           set(CAPNP_EXECUTABLE "$ENV{CAPNP}")
@@ -33,7 +32,6 @@ if(BUILD_TESTING)
         endif()
       endif()
     endif()
-    endif()
 
     set(CAPNP_INCLUDE_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
     #TODO(someday) It would be nice to use targets instead of variables in CAPNP_GENERATE_CPP macro