Remove useless import of pushout in number_field.py.

Author: Jean-Pierre Flori

src/sage/rings/number_field/number_field.py

@@ -100,7 +100,6 @@
 from sage.misc.functional import is_odd
 from sage.misc.misc_c import prod
 from sage.categories.homset import End
-from sage.categories.pushout import pushout
 
 import sage.rings.ring
 from sage.misc.latex import latex_variable_name