HPCC-13858 CMake not detecting MySQL include directory

Take out line that sets MYSQL_INCLUDE_DIR to empty string,  That allows FIND_PATH to set it in FindMYSQL

cmake_modules/commonSetup.cmake

@@ -668,7 +668,6 @@ IF ("${COMMONSETUP_DONE}" STREQUAL "")
         endif()
       else()
         add_definitions (-D_NO_MYSQL_REPOSITORY)
-        set(MYSQL_INCLUDE_DIR "")
       endif(USE_MYSQL_REPOSITORY)
 
       if(USE_APR)