Do not leak an internal variable from FindSphinx

An unused initialization of the theme import result remained defined in
the caller's scope whenever no theme was checked, for instance without
a Python interpreter or without theme components.

Drop the initialization.

Change-Id: I46beb231e1e05e057e810edeb911fdfcfa15cf48

cmake/FindSphinx.cmake

diff --git a/cmake/FindSphinx.cmake b/cmake/FindSphinx.cmake
index b80fd2d..085b72a 100644
--- a/cmake/FindSphinx.cmake
+++ b/cmake/FindSphinx.cmake
@@ -83,7 +83,6 @@
   unset (_Sphinx_VERSION_STRING)
 
   find_package (Python COMPONENTS Interpreter)
-  set (_Sphinx_BUILD_RESULT FALSE)
 
   if (Python_Interpreter_FOUND)
     # Check for Sphinx theme dependency for documentation