Revert "find sphinx-build before calling build.py"

This reverts commit 07200f445a because
the latter doesn't work with virtualenv correctly.

doc/CMakeLists.txt

@@ -4,13 +4,8 @@ if (NOT DOXYGEN)
   return ()
 endif ()
 
-find_package(PythonInterp
-  QUIET REQUIRED)
-find_program(SPHINX_EXECUTABLE
-  NAMES sphinx-build sphinx-build-3)
 add_custom_target(doc
-  COMMAND ${CMAKE_COMMAND} -E env SPHINX_EXECUTABLE=${SPHINX_EXECUTABLE}
-    ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION}
+  COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION}
   SOURCES api.rst syntax.rst build.py conf.py _templates/layout.html)
 
 install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html/