Remove old _function_arg_update attribute when setting DirichletBC value

[jrmaddison]

Description

Fixes the following behaviour:

bc = DirichletBC(V, expression_depending_on_u_to_be_interpolated, sub_domain)
old_value = bc.function_arg
bc.homogenize()
u.assign(new_u_value)
new_value = bc.function_arg  # Modifies old_value

The second access to bc.function_arg modifies the value obtained in the first access, which seems inefficient/unintended.

Caused by an old update function, bc._function_arg_update, being retained after the bc.homogenize call.