SQLAlchemy: Remove CrateCompiler.prefetch class variable

It has been added with GH-188 without further reasoning.

Author: amotl

src/crate/client/sqlalchemy/compiler.py

@@ -208,8 +208,6 @@ def visit_ARRAY(self, type_, **kw):
 
 class CrateCompiler(compiler.SQLCompiler):
 
-    prefetch = []
-
     def visit_getitem_binary(self, binary, operator, **kw):
         return "{0}['{1}']".format(
             self.process(binary.left, **kw),