[lldb] Remove unimplemented SBTraceCursor::GetExecutionContextRef

Fixes TAPI verification error:

  SBTraceCursor.h:130:23: error: declaration has external linkage, but dynamic library doesn't have symbol '__ZN4lldb13SBTraceCursor22GetExecutionContextRefEv'
  SBExecutionContext &GetExecutionContextRef();
                      ^

Remove the method as it doesn't have an implementation.