[krbdev.mit.edu #1685] consider calling remove_error_table() in krb5_free_context()

[Tom Yu via RT]

As noted in #1665, krb5_init_context() and krb5_free_context() can leak
error tables.  The fix in #1665 prevents multiple registrations of error
tables, but krb5_free_context() does not actually remove error tables. 
We may or may not care about this.