build: removed some unused imports found by pyflakes

thanks to Jelmer for pointing me at pyflakes

buildtools/wafsamba/samba_asn1.py

@@ -1,6 +1,5 @@
 # samba ASN1 rules
 
-from TaskGen import before
 import Build, os
 from samba_utils import *
 from samba_autoconf import *

buildtools/wafsamba/samba_errtable.py

@@ -1,5 +1,5 @@
 # waf build tool for building .et files with compile_et
-import Build, os
+import Build
 from samba_utils import *
 
 def SAMBA_ERRTABLE(bld, name, source):

buildtools/wafsamba/samba_optimisation.py

@@ -6,9 +6,8 @@
 
 # overall this makes some build tasks quite a bit faster
 
-from TaskGen import feature, before, after
+from TaskGen import feature, after
 import preproc
-import os
 
 kak = {}
 @feature('cc', 'cxx')